we cut hdpe on machines using high speed steel cutters and also carbide cutters so we just blitz it, but i reckon that you should go with a coarse blade run slow rather than a fine blade run fast. dont let the blade dwell too long on the cut or it will heat up and blunt too quickly.
as for water, be wary when using electric tools ....


i'm sure others who've manually worked hdpe will chip in also