In the cockpit
We made one out of a left over piece of A grade plywood. It fit in the center of the cockpit over the cockpit. When not using it we stowed it flat up against the fore and aft bulkhead under the cockpit on the starboard side. So when not in use the thing took up maybe 3/4 of an inch of space that really made no difference. This however limits the length and width of the plywood.
Really light jobs such as a simple soldering generally get the suction vise.. It works really well on the formica of most yacht tables.
I also have, on a number of boats, drilled four holes in a chest level stair on companionway ladder and mounted a vise on that... Now to keep from screwing up the stair while working there is one of those flexible cutting boards that goes between the vise feet and the stair; which also has the same holes cut in it. This is not shown in the photo for clarity reasons. For little jobs such as screwing small parts together, drilling a few small holes, basically 99% of the projects on the boat the little vise on the stair works great, for bigger jobs the piece of plywood in the cockpit, with the same 4 holes in it works well. Bigger than that and you should take it to shore if you can.
Now holding the plywood down in the cockpit was the task of the assistant. The assistant varied by the job at hand. If it was a job that required some heavy wrench work then you need to pick a heavier assistant. If it was a light job that was messy, then you could pick a svelte, good looking assistant to provide scenery for the job.
No one ever notices the 4 little holes in the stair till I point them out... So I don't think it detracts much if at all from the fit and finish.
Guy