probably seized piston, my trs ram was seized i soaked it in blaster for a day and used a gear puller (gently) to remove it, cleaned it up, and installed new seals.

What year boat? Mercruisers before 1986 have a prestolite pump. If so it looks like a box with can on top. These pumps have the hoses on the bottom of the pump instead of the side on Oildyne pumps.
If your pump is Prestolite and the trim works fine until it hits top whereupon it locks up, the lower one inch on the bottom of the pump called the control valve is bad. Just buy the part from Mercruiser and a kit of 4 'o' rings. Be careful the four bolts are bolted into alum, do not over tighten.
If you have an oildyne pump you have a down relief valve leaking.