Feasibility of reconstituting existing 5083 Aluminium

Can you melt 5083 and cast it? yes, this alloy is ok for this role. It is not heat treated, it has somewhat good protection against environment.

I worry more about thin shell casting problem - metal may start to solidify in place before filling the mold with a size of a boat fully. Especially if you plan partial fill, using several pouring steps. And I assume you are not planning to heat up the mold. So, I think this will be the real problem, not the alloy.

Aluminum is usually melted in a protective atmosphere to reduce oxidation ; even more important with the Mg alloy content. Melting and casting aluminum is not a good backyard project. A charcoal bed on top of the melt and the right flux would make it possible although strength is likely to be reduced by loss of Mg. You would need to research the flux and maybe special treatment of the mold to reduce oxidation. The traditional high silicon alloys ( like 10 % Si ) would be better candidates for armature foundry.