That is a lot of water coming from something. Drain Plug leaking ? Or 20 year old rubber failing.
Do not fill a boat with water. Keep it out of the boat.
Power trim has a place with a 50. Back strain is real with them.
If the pin does not have a chain on it ???
Be careful not to drop it. Replacement might be difficult.
Use post # 14 ! It is foolproof. No simpler way for a set of valve cover gaskets. Do it.
Do turn the crankshaft in the correct direction..
Remove ALL the sparkplugs first ! To make turning the engine enjoyable.
You can use the same stuff as used in floating docks. 12 " thick. Closed cell. Used to be colored blue. Ages ago.
Get out of the old ruts of doing things.
Why are you sweating about doing foam on the bottom ?
My small Aluminum boat has all the foam in the space under the full width bench seats. Do some L X W X D to see if you can get enough floatation there and under any raised decks.
Plain old Closed Cell foam block would work Since gas and E10...
Buy new plugs AND reset the gap to what it should be. If you break the white insulator ? Have to replace it.
Has any ignition parts been correctly replaced ?