The only way I know how to do it, is to capture or buy a planet with one.

You cant make a seaport in space, the only way to do it is to capture or buy a planet that
already has them.

If your colony is in a natural basin(a place were water will fill when atmosphere is
raised)then as the water level raises the colony will but not the land around it. I have
been able to recreate this 3 out of 5 times, but it really accounts to luck of the draw
and good guessing.