I've got a 16ft aluminum boat with a 70hp evinrude(weighs a few hundred lbs)... but my boats not all that heavy...

loaded with duck decoys, coolers, gas, guns, etc, etc my 3.0 3.73 pulls it just fine.

i've pulled a good sized glastron fish and ski(20ft w/ 5.7l motor) and it pulled it alright... but it was a dual axle trailer (which helps a little)... i wouldn't want to be doing it all the time.

my truck and boat:

 

well i got the boat home and i have to say a was very happy with how well it towed. i was doing about 3300 rpm at about 70mph. unless i was at a dead stop it didn't even feel very heavy at all. i'll try to get a pick up this weekend.