What do call a boat with two hulls?

Oil tankers frequently have two hulls, one within the other. This contains any leakage of cargo or pprevents collisions from reaching the cargo area.

A catamaran is a two-hull boat with a raised platform between them. Catamarans are generally fairly small and sail-powered.

If it's a larger ship with two thin hulls and a larger tubular portion below the water, it may be a SWATH - "Small Waterplane Area Twin Hull"