The best materials to use for a pipelauncher are those with highest strength to weight- aluminum, or titanium alloys. But if you want to build a small pipelauncher, steel might be the best choice in terms of cost, but there are a lot of advantages aluminum:
In kastenmarine it discusses advantages of using steel vs. aluminum in boat design. The type of aluminum used for these marine uses is "Aluminum 5083 H-116 plate... has a yield strength of around 34,000 psi and an ultimate strength of around 45,000 psi". Though according to article welding this stuff significantly reduces it's strength to: "23k psi yield, and 39k psi ultimate strength"

Probably the best place to build a pipelauncher is in a drydock. But something 1000 meters long is probably too long to fit in any drydock, so pieces could be built and these pieces can mated in the water.

So Distance = acceleration 1/2 times Time Squared
So 500 meters = 10m/s2 1/2 times 10 seconds squared. Which means if you accelerate slightly faster than 1 gee [10 m/sec per sec for 10 seconds in that time you will travel 500 meters and have reach a speed of 100 meters per sec [360 kph or about 223 mph].

Starting from a simple shape: 50' diameter 1" wall with
Aluminum 5083 which is 1640' [500 meters] long. So 157' by 1640' is 257480 sq' of 1" plate. Which is 21457 cu' times 168 lbs which is 3,604,776 lbs. So 50' diameter has 282,743 square inches. This gives 12.7 psi. A 50' diameter pipe 1" wall with Aluminum 5083 at weakest from weld of 23,000 psi yield would be 76.6 psi. Calculator:
http://www.dixiepipe.com/barlows.html

Now this gives a general idea or framework. Since each 100' section will be about 16th of overall length. The weight these sections should around 16th of total weight of 3,604,776 lbs [225,298 lbs].
Starting from top. Need a deck in which rocket sits is launched from.
The biggest rocket ever made is Saturn V, it's diameter at base was "Diameter: 10.06 m (33.00 ft)" So a deck of around 60' in diameter, should be big enough. Below this a hundred foot section which is 50' in diameter- it would have a circumference of around 157', evenly space around this have 12 6' diameter 1/2" thick Aluminum 5083 walled pipes. In between these pipes have 1/4" Aluminum 5083 plate. These spacers will be about 7' wide.
This 100' section weighs:
6' diameter 1/2" wall 100' long has 78.54 cubic feet of Aluminum 5083 weighing: 13194.68 lbs. So 12 of these pipes weigh 158,336.2 lbs. Spacers weigh 29,400 lbs.
The deck itself will made of structural Aluminum 5083 plate roughly equal to 3" plate disc 60' diameter: 118,752 lbs.
So total: 306,488.3 lbs.
In this top section stuff can be stored- such as topping off propellent for rocket, liquid air and fuel for pipelauncher, generators for any power needed, electronics/communications, etc. The pipes and other spare area can be sealed [filled with helium to enough pressure to resist exterior pressure from ocean depth- with a safety factor this at most might be around 200' [6 atm]. Just for reference these pipe at 23,000 psi yield have max pressure of 319 psi.
Oh and for further fun, if these pipes were fully pressurized. They at each at their end have 4071.5 square inches- 12 having in total 48,858 sq inches at 319 psi this force equals 15,585,704 lbs. Hmm this isn't as dramatic as I thought. The Saturn had gross mass of 6 million lbs- and at 1 gee the Saturn would weigh 12 million lbs. But if pipes had a seam which built up where ever it was welded so allow you to use the unwelded str yeild of 34,000 psi, you would have 472 psi. But I will grant that accelerating a Saturn V mass vehicle at 1 gee would get closer to the structural strength than would be comfortable. Maybe, limit it to 4 million lb vehicles such as the Shuttle size.

Ok so that top section is being pushed [has compressional loads, the next section deal with generating the power and most of structure will be pulled [mostly tensional loads].

The next 100' section down can be called the firebox. It will have the plumbing that feeds liquid air and fuel to sprinkers/burners, a cap which tops the firebox, and access for maintenance. The cap will at top and since heat rises it would be hot if not cooled.

The basic structure of top section will continue most of firebox section except the 10' section which mates with the cap. This 10' section will probably always be dry [not submerged in sea water] and though the changing pressure might a serious problem the heat and cold could be endured by a person in the right location in this area because of the relatively short period of time in which they occur. The top 10' section will made about two times stronger- with two pipe sections one 50' diameter 1/2" Aluminum 5083 and one 38' in diameter 3/4" walls. They need to be made stronger because they going to breached and should be breached in areas which less load bearing sections [not over the 12 6' diameter pipes]. At base of 10' section will a ring- 50' outside diameter 30' inside diameter made of 1' Aluminum 5083 plate [this includes a 4' catwalk]. This ring ties in the section with 12 6' diameter pipes and spacers- this ring will have 12 6' holes above the pipe.
So the roof of this second section will be 2" plate 50' diameter: 54,976 lbs
10' pipe 38' diameter 3/4" wall: 19,996 lbs
10' pipe 50' diameter 1/2" wall: 17,541 lbs
Ring: 50' outside 30' 1" plate: 11,702 lbs

6' diameter 1/2" wall 100' weighs 13194.68 lbs, 90' is 11,875 lbs. And instead of 1/4" spacer, I want 1/2" for this section. So 90' of 12 pipes weigh: 142,503 lbs and the 1/2" wall spacers weigh 58,800 lbs for 100' and 52920 lbs for 90'.
So the second section weighs 299,638 lbs.
Add this to top section 306,488.3 lbs gives total of 606,126.3 lbs.
Next two section are same each 100' section weighs:
158,336.2 lbs lbs pipes
58,800 lbs spacer

total: 217,136.2 for each 100' section
Total: 1,040,398.7 lbs for the 4 sections.
The next 5 section will be a single pipe 50' in diameter with 1" inch wall. As calculated above each 100' section would weigh 219,800 lbs. Total: 1,099,000 lbs
All 9 sections: 2,139,398.7 lbs
Next 5 sections will be a single pipe 50' in diameter with .75" inch wall [164,850 lbs per section]. Total: 824,250 lbs
14 sections: 2,963,648.7 lbs

Next 3 sections 50 diameter .5" inch walls 109900 per 100'.
329,700 total.
Grand total: 3293348.7

Add about 10% and it's close to the 3,604,776 lbs number given in the beginning.

Now imagine you could drop this from a vertical position in which the bottom is 50' above the water. First second it falls 4.9 meter and by end of 2 second it falls 19.6 meter. So within two seconds the open end of bottom of pipelauncher hits the water. At end of 2 sec it's fallen about 74', so the end is 24' under water. Up until this point the water as not significantly slowed in it's free fall and is traveling at 19.6 meter per sec [about 44 mph]. The air pressure inside the pipelauncher is about the same same the outside air [14.7 psi]. But as it continues to freefall pressure inside will increase the deeper it falls. By end of second 3 it will have fallen in total 144' or 94' under water and have a speed of around 65 mph and air pressure inside pipe will around 16 psi and water inside pipe will depressed about 3' lower than sea level and you will force of around 1/2 million lbs. So the free fall will braked so have about 6/7th of 9.8 meters per sec2 or about 8.4 m/s2. By end second 4 the force will increase to around 1 million lbs of force and the pipe will be around 200' under water depressing the water around 6' under water with inside psi about 3-4 psi above gauge. In 4th or 5th second air vents should be opened in fire box area for a few seconds then closed again.
By time pipelauncher drops so it's end is 800' below surface the air pressure could around 1 atm over gauge and since any pressure over 12.7 psi will cause pipelauncher to accelerate upward- the speed that's it's going down will be decreasing. When water level is approaching the bottoms of the 6' pipes pressure can be maximumized say around 20 psi and value at top if pipes can be closed, thereby allowing one to charge them with air pressure.