So I took out my new to me boat today (1978 centurion 21ft I/O) I know not a jet. Lol but it got hot on me and started spraying water/steam from a hairline crack on both exhaust manifolds. 1.... Are these repairable (weld?) 2.... Will the cracks alone cause heating issues?

is it a crack, or corrosion causing a pinhole?  might just be a better idea to get a set of new ones.  try cleaning up the area with a wire wheel and see whats actually left in that area.  id be worried if it was corrosion being on the inside of the manifold as well, spraying water near your exhaust valves and possibly causing engine damage.  pull em off and figure out what the cause is.

and post pix!  we love pix.

It's a sbc 350

as long as its a crack i do beleave they are repairable, but they are very limiting on power for your motor.id look into something a little better.