Agitation won't increase your attenuation. In general, outside of
natural convections inside your fermenting vessel, you don't want to
agitate things. Flocculation is based upon having a still fermenter.

If you resort to malt extracts and dried yeast, you'll go back to
buying beer.

Spent 10 years in the brewing industry after I was a homebrewer, and
consulted in Scotland, US, Canada, Zambia.

The heat alone that you need for a proper boil would bring a lot of
danger to your boat. If you are willing to do this to save a couple
bucks on beer, then you might want to revisit some safety rules.

Honestly, research what is available and get some kind of bulk delivery
when you are there. You can order these things online in the UK, and
they're great.

Cheers