Ah, I just necro-posted in an older thread before I saw this one. Just finished a year-long refurb of my 30-foot sailboat and trying to figure out where a tank rack & etc, and maybe someday a compressor, could go.
As others have said, for a day-trip, a small inflatable is the way to go. But I'm thinking of those week-long live-aboard trips, where you can surface, stick the hot water hose into your suit (ahhhh!) and have the coffee waiting. Running back to port for an airfill at 6 kts probably limits the ratio of sailing to diving though.
As far as getting back on board, on friend's boats, we always just left several lanyards hanging over the side. Clip your gear onto a lanyard and shrug out of it, then climb the ladder unencumbered.