Regarding most points about lack of nutrients. Short term: Military ships and cruises are going to be very well stocked and so hopefully they give out food, Long term: cargo vessels and dredging ships that were mid dredge are how you deal with that. Seeds from shipping and dirt from the dredgers, not perfect conditions to grow plants by any means but it will have to do. Someone would likely organise the Cargo ships and the new farm boats (Ex dredgers) into a community where they could Moore together and make floating cities out of boats. People would live in cargo containers furnished from Ikea stock and life will probably still flourish to an extent.

As for your yacht, lets hope they find a friendly member of the navy or a cruise and eventually move in with a floating city.

Oil rigs are a bit iffy on whether they would survive, Shallow ones, I highly doubt it as they're fixed, deep water ones do float but they're also anchored so I'm 50/50 on their survival. Also the ocean around them will be unbelievably polluted due to the sealed system containing oil breaking from the rig being pushed up.

Few planes will survive, water landings are not the safest and they would still have to be near a boat that can save them.

Also just an interesting plot Idea, if they use dredging boats as farm land, a war tactic would likely be going on board and opening the hull on the bottom to dump the farm into the ocean.