Unpowered yellow submarine used for torpedo target

This test platform, formally titled Weapon Set-To-hit Threat Target (WSTTT), was developed in San Diego to support Operational Test and Evaluation Force (OPTEVFOR) operations used to test Anti-Submarine Warfare (ASW) sensors and weapons. It can be lowered and raised from a surface support platform, but has no dedicated propulsion system. The platform allows unarmed ASW torpedoes to impact the vessel without damage.

Possible picture here.