: Installing foam bulkheads seems easier, lighter, & quicker than plywood,
: however I am concerned over their watertightness. Are the foam bulkheads
: watertight if well caulked?

Yes they are. Use a good quality adhesive and sealer. I used a urethane adhesive and sealer on my fiberglass boat and had no leaks at all, plus the joint is actually stronger than the mini-cell so failure won't be a problem.

Ian