I don't normally see steam coming out of mine. On the few occasions that mine has started to steam and engine temps climb to 180, I shut the engine down and check the intake filter right above the seacock. That gets filled with weeds and restricts the flow. To empty it, first shut off the seacock, unscrew the filter, remove the screen and dump it out, I then like to open the seacock to make sure I'm getting good flow. On a couple of occasions I've had to remove the filter housing, open the seacock and push a piece of wire down and out the bottom of the boat to clear lodged seaweed. It's possible to suck other debris up there as well, like plastic bags.
Also as Fortitude stated, check the impeller while you're down there.