I thought I'd post a solution rather than a question for once. I have just finished mounting my bottle rack to the deck of my boat as part of a major over-hall. The problem was, as usual, no access under the deck and no desire to put in two inspection hatches so I came up with a plan.
1. Cut a strip of 316 stainless. This bit is about 60 x 20 x 6mm. Drill and tap an M8 and an M4 threaded hole in the centre.
2. Drill a series of 8mm holes in the deck to cut a slot approximately 8mm by 21mm, so the plate just fits through.
3. Push plate through the hole with a bit of string or thin cable through the larger tapped hole (so you don't drop the plate).
4. Pull string/wire and pull plate flush to underside of deck. Using a fine screwdriver or pointy thing, position the plate and insert an M4 bolt. Tighten to secure plate in place. This wasn't the strongest fix so be careful. A washer would help but would leave the bolt head proud of the deck.
5. Insert bolt. I wrapped the bolt in plastic then filled the remaining slot with epoxy to seal, but mastic type sealant would do. The epoxy did help grip the M4 bolt and retain the plate a little better. The setup shown was to lift the plate to the underside of the deck but still allow access to get the resin in.
The plate was not greatly attached to the under side of the deck and rough inserting of the M8 could push it off, but once gripping, has given a solid connection. I was able to get about 28Nm torque tightening the M8, so a pretty solid connection and it will not pull out.
I am sure the method can be improved upon but it has worked well for me.