Yes, I am a retarded,,uh, I mean, RETIRED Coastie icebreaker sailor. We learned to reheat the core by warm drink, hot bath (body in the water, legs and arms outside of the water) or strip them & yourself and get under the covers or in the sleeping bag with them to re-warm them.
From the time you rescue them, do not make or allow them to walk,,,,carry them to a warm place & start warming them up without causing them to exert/expend any more energy.
First get them into the warm environment, add some hot drink (cocoa or broth preferred), while preparing the means to rewarm them via bath, microwave warmed towel wraps, warm bath (appendages out of the water), body to body heat transfer.
After they have been sufficiently re-warmed, spoon feed them some more broth, hot cocoa, etc to replenish their energy levels, & they will start making their own body heat again.
Don't forget about the mammilian diving reflex, it makes it possible to save someone but it also means that the majority of their bodily functions have shutdown. After re-warming the person, you must spoon feed easily convertible energy into their body until their body's sub-systems come back online.