Sounds like your holding back on the backroll. An unconscious decision, lot of boarders deal with overcoming this problem one way or another. I had the same problem with tantrums. Solution as follows:
You need to make a deal with the guys on the boat. Every time you do something stupid, like the problem mentioned above, someone on the boat gets a free shot to your shoulder. Sound retarded, but I guarentee results. For this to work you have to be more afraid of getting punched by the tough guy on the boat, than crashing on a big backroll.