Good plan.
Keep in mind that unless you're actually on the beach, or within a mile or so of it, even a huge wave will flatten out pretty fast. If you look at actual films of tsunami strikes, it isn't the giant breaking wave you see in the movies, it's a sudden inflow of water like an instant raging river going down the streets. Any reasonably tall object that is strong enough to withstand heavy water flow and especially impacts by floating cars and such will be a good bet. A hill, of course, is best, but a large modern building would work. No need to find a makeshift submarine.