Please help with my Merc Issues!!!

[1BadTitan]

I have an 85 Merc BlackMax 150. It idles perfect on the trailer all dayat about 1100rpm. As soon as I put her in the water, the idle drops to 400-500, starts to lobe and stalls. Also, when I'm cruising out of the no wake zone, (about 10mins) it seems to load up and bog, but as soon as I tilt the motor up some, I can get it to rev up and go. Sorry if I'm all over the place here, but I cant figure it out and am about frustrated at this point.