Just read a recent post about the detailed spec's towards the GMC Terrain.
This is what i found
Trailer Towing* Maximum (lb./kg):
3.0L V6 = 3500/1588
2.4L I4 = 1500/680
Hope, its big enough for your Boat/trailer.
If anymore info is needed check out this guys thread, has some pretty usefull info about the terrain.