Per code (US) 30A requires at least 10AWG wire. On a boat
my preference would probably be to go up one size (8AWG) simply because corrosion
gets at things and it's nice to start with a little buffer. Typically multi-strand is used on boats for the added flexibility. Tinned copper wire is used to reduce corrosion
. Vinyl insulation
is the most common in 'marine' wire.
The common 'marine' wire is Ancor or similar, they make a multi-strand, tinned copper, vinyl insulated product for the marine
market. I generally use MTW (machine tool wire) or similar generic products that are tinned copper, stranded (some MTW is extra fine strands which may be more of a corrosion issue - care in selection required), vinyl insulated - usually at a significant savings over Ancor, especially when purchased at a wholesale house.