Yes, that is the seawater pump. It wont turn by hand all that easily as you are trying to compress the rubber vanes in the pump....Put the belt back on, check for proper tension and watch all the pulleys to see if they are turning and if they are still in line with each other....
Thats great news. I really didn't want to drop 6 bills on a new impelller and housing. I had to grip it pretty hard to get it to spin. The tensioner stud is located on the alternator bracket right? If this is it, it's turned in about 75% so I guess the belt has seen it's best days already.