Make sure that you know for sure whether the leak is between the plexiglas and its frame or if its leaking around the hatch base itself.
I re-bedded the plexiglas into a frame many years ago on a Sea Ray. It was pretty simple, I used a black adhesive caulk which I think was 3m 4000. Of course, you'll want to clean the area as best as you can before re-bedding & then clean up any excess before it dries.
Use soapy water and a garden hose around the base of the hatch first. Keep the plexiglas dry while doing this. The soap helps break the surface tension and if that is the problem it should leak immediately. If you don't get a leak there move on up with the soapy water and hose and flood the Plexiglas in small areas. That should locate the problem. Maybe try covering the Plexiglas seams with electrical tape and see if that stops the leak. Also check that the hatch dogs pull it down tight and squeeze the gasket. Capt. George
What year is the 38 and do you have a Bowmar or Maxwell hatch?
Maxwell used some type of black sealant that seems to remain uncured and in the heat will actually expand and drip from the below the hatch.
I agree with all of the above suggestions and make sure of where the water is coming from first. If it is the seal between the aluminum and the acrylic, remove carefully, clean up the old adhesive (be careful as caustic chemicals will destroy the acrylic) and reinstall using a silicone based black caulking. Pre-tape the hatch when using the caulk, so your caulk lines will be perfect.