@[email protected] drop a magnet in a cylinder with a portal at the top and bottom, and you have a free infinite-power generator (assuming the portals take nearly no power, and you’re in an accelerating reference frame)

Take your >1g water-torch ship, use it to kickstart the generator above, use the power from that to accelerate ions in a cylinder (with a portal at top and bottom). Eventually, your ion-based thrust is enough to cutoff the ocean portal. Reactionless thrust! + Particle beam cannon on-demand