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.