Concept suggestions needed

Hello builders world…i’m newbie John from Greece

I start making a project and i need your thoughts…

Watching this instructable video

i decided to make a small sea scooter board,on this board:

:rofl::rofl::rofl::rofl::rofl::rofl:…but i dont know many things so i need your help.
I bought from their suggestions the following:

1.DT700 brushless motors: 2 pcs (i will run them parallel always at 30A,the easy way is to stuck the potensiometer to 30 A and use an run -off switch for the hole system but i dont know where to connect…Am i correct or …

  1. Kort nozzle for DT700: 2 pcs

  2. ESC speed controller 40A with BEC(i think from the price is garbadge…)


  4. 10K Linear Slide Slider Electronic Potentiometer Module Dual Output Arduino

  5. Sony Konion US18650VTC6 3120mAh - 30A.I think that 3s10p is a good choice for about an hour runtime,

My questions: i need fuse?what A and where to place it?
2.which waterproof on -off switch and where to connect? it safe to use those servo tester, potensiometer and esc?or sth else?
4. What bms?with or without ballance etc.
5.all in a ip67 box or separate the li-ion pack from the esc?
Thanks in advance