Or heater core? Lots of threads on this here on WW. Did it myself a year ago. And just walked my neighbor thru it again a couple weeks ago. Pull the heater out of the boat. Pull the core out of the metal box. And take it in to your neighborhood auto parts store (Napa in my case). They compared it to several cores & found the right one. It's like a 90s Mustang core. Cost $19. Works like a charm.
Then trace the heater hoses to the engine. At their lowest point in the bilge (where you can reach), install a pair of the radiator flush Ts in the lines. Then when it comes time to winterize the boat you just have to pull those two caps off & the heater & part of the engine will drain into the bilge.