Assuming you're insulating the boats living space (as opposed to the
refer / ice box)?

Yes, the Big Box home stores often have extruded polystyrene. (These
[nominally 2 x 8 or 4 X 8 sheets] are 'rigid foam' not at all like the
'cotton candy' fiberglass batting you may be thinking of, and are
available in thicknesses from 1/2 to 2" at your favorite home store).
There are two manufacturers: Dow's 'blueboard' and Owens Corning pink
'Foamular'.

Extruded polystyrene will not absorb moisture over time, and has the
same 'R' value on day one as on day N times 1.

Other 'foams' - and there are many (!) - may have a higher insulation
'R' value, but may not be as suitable on a boat because they're
somewhat hygoscopic and their insulation value will decrease over
time.

Extruded P. will 'melt' like the Wicked Witch of the West if it comes
in contact with solvents (Acetone, rubber cement, polyester resin), so
attaching it to a hull side (for example) may take a little
creativity.

MW