A system and method for measuring fluids include a vessel containing at least two fluids with a drain pipe, a seal housing, a tool assembly, and a float assembly. The seal housing fits onto the drain pipe of existing vessels or tanks, and the tool assembly and float assembly are inserted through the seal housing to the vessel to measure the fluids inside the tank. The tool assembly has a shaft section and a flexible section. The flexible section extends into the vessel and through any bend in the drain pipe so that the system is compatible with retrofitting vessels with different drain pipe shapes. The float assembly can be released from the end of the bent drain pipe to float to the fluid boundary between different fluids in the vessel. The float assembly can also be retracted to safe position within the seal housing.