so basically i could do that but there are some issues which should be cleared up before:
shipping: as a private person it’s legally not allowed to ship li ion batteries this big, even within germany. Therefore i use UPS because it offers an option for li ion batteries not to transport by airplane. I just checked that this also works for spain and would cost around 30€
cell configuration: which samsung cells do you mean? the most common cells are samsung 30q but they went pretty expensive and are just capable of 15A discharge current. So a 6p config can handle 90A which sounds a bit low for an electric powered watercraft and the capacity of ~840Wh sound also a bit low (most efoil builders are using 1300-2000Wh).
BMS: do you have a certain BMS in mind? the most builders are using splitted batterys (like 2x6s, 2x7s or 2x8s) with a “hobbystyle” balance plug instead of a BMS. I made a few 30Amp li ion batterie packs for eskates with LLT-Power Smart-BMS which worked great. They also offer a “up to 200A” Version which sound nice for this project
housing: using ip68 plastic cases is a common way, a 14s6p 18650 battery with BMS could fit in this one, for example