“The battery cells we sell will store A LOT of energy (that is the whole point) – more than enough to maim and/or kill you, your friends and family, and burn down your house, RV, boat or whatever, if you do not know what you are doing.”
If you don’t know what you are doing then the choice is simple. Buy your batteries, do not build them.
Why? A car battery is dangerous, but there are only two terminals to worry about. When you build a LiFePO4 battery from cells, the basic 4 cell configuration involves 8 terminals and multiple other live connections on the BMS. You will be moving these around, assembling (and disassembling) them several times. There will be many opportunities to mess things up, get careless, and get hurt, even knowing what you are doing.
Build your own LiFePO4 battery if:
You plan to keep the boat for more than 3 years.
You need more than 100Ah of capacity.
You want to save some money. (You can build a battery pack for about half the cost of a top rated, fully assembled, ready-to-drop-in battery.)
I could write forever on this, and many have. I recommend watching Will Prowse on Youtube. He has excellent information and advice on all of this, including how-to videos.
Good luck and be safe!