I assume if the base hull was a WWII LCM then they weren’t anywhere as near armoured as they look?
From Wikipedia:
High-hardness XAR-30-type steel and bar armor provided ballistic protection for the crew from rounds up to .50-caliber in size and offered some protection against high explosive antitank rounds up to 57mm. Below-waterline hull blisters provided added hull protection, minimized draft, and increased stability. The Tango’s armor, however, generally could not withstand the full force of a B40 rocket