It's relatively easy to re-cast aluminum at home at small scale. I think the biggest challenge for boat-sized would be the massive amount of energy needed to heat all that metal, plus the huge crucible and machinery to pour it.

Another issue with home-cast aluminum is alumina (oxidized aluminum) which will make small inclusion defects in your print. I've had several casts fail because I wasn't able to properly separate the alumina and the high-alumina areas were very brittle. I think most aluminum foundries have the electrolysis capabilities to convert all the alumina back to aluminum, but this is an extremely high voltage hot process that does not seem feasible for garage-scale.

(As an aside, aluminum cans have a ton of oxide because of the high surface area, so you get a ton of dross that must be skimmed off to use the molten aluminum. We would do it in two steps: first melt cans and pour in a muffin tin to make small ingots, then use those higher-purity ingots to cast the desired piece. Probably better in most circumstances to just start with lower-surface-area aluminum materials: better to pay for quality bar stock or ingots rather than spend the same on more charcoal for multiple runs.)