Red oak has roughly (very roughly) twice the strength of WRC, and is also roughly twice as heavy. So if you halve the sizes you should end up with nearly the same strength and weight. That said a stringer 3/8" square won't be very strong regardless of what wood it's made of.

Red oak will turn black and eventually slimmy when it gets and stays wet, not a nice wood for boats unless it's sealed somehow.

Bill H.