Slowing the rate of time on the ship could allow any distance to be traversed. The 'slow boat' would behave as if it were going at relativistic speed, even if it weren't. An outside observer would see the ship's lights wink out as it sailed off into the distance. From the ship's observation deck, passengers would see the stars suddenly start shooting past, hundreds of millions of them streaming by, and in the distance the swirling center of the galaxy would be visibly circulating like a whirlpool vortex.

Generating an intense gravitational field around the ship would have this effect, such as on the surface of a neutron star, or black hole. But that would be fatal, and would require the mass of the ship to increase extraordinarily. In addition, increasing the mass as such would reduce the velocity of the ship to almost nothing, via conservation of momentum.

Another way might be to cause all the matter in the vicinity of the ship to behave like a Blake-Sherman (BS) condensate, where the speed of light is reduced to almost nothing. As such the properties of all things electromagnetic, including molecular processes might be similarly slowed. The intergalactic BS condensate ships could travel anywhere in the universe, in subjective ship time, using little energy (slowboat), and without damaging exposure to blueshifted radiation (non-relativistic). For the duration of the trip, passengers would enjoy a surreal true-color view of the galaxies, without any viewing aids, while inhabiting their own temporal BS frame of reference.