Submerged motor and foam casting of today's blocks. I am told the metal absorbs the salt water. True or not?

If you're talking about parts made using the lost foam process, then no, they should not absorb salt water. Porosity is an issue, but when needed the parts are impregnated with resin to seal them. Typically it's not required. Outboards have lost foam blocks, exhausts, etc. so the block is always full of water while the engine is running, and the exhaust housing is under water. The gearcases are usually die cast. Anyway, short answer would be not true.