some vehicle do not come with it. its just that way. if your boat is a small aluminum boat the vehicle wont notice it back there. if its a boat that weighs more that 1000 lbs. tow it in the next gear down, this will keep the tranny from "hunting" for a gear when going up slight inclines which will heat up the tranny and destroy it. if you do not have an external tranny cooler, I would suggest getting one. It will save your trannys life.
The '08 SE, SE-OR, and LE have a new shifter that ends at D, then has another gate to the side with a +/- for manual shifting. There is no button on the side. Only the S model like mine still has the old shifter.
I can tell you that I tow a 2,000lb Starcraft in OD, and it doesn't seem to bother it. I only downshift to maintain control on small twisty roads. The five speed doesn't seem to hunt like older trannys. Since it is the same transmission as in the Titan and Armada, I think its probably pretty stout.