The easiest most durable is to put float bags like they use for Lasers in the hulls.

The float bags are $16 for 50 lbs floatation & $35 for 250 lb floatation

An alternative is a bunch of inflatable beach toys. Stuff them in the porthole and inflate them. They are on the clearance rack this time of year so, get the best ones you can find. I have seen some "heavy duty" beach balls, that would be great, for a couple of dollars.

Now for really cheap:

* Super heavy duty trash bags (tie and glue the ends shut)
* Collect old Lifejackets from your friends. Be Sure to get a throwable, so he will always be legal (3 in each hull should do it)
* empty plastic bottles (1 & 2 liter soft drink and 1/2 gal milk)
* Search the beach for Styrofoam crab trap floats. (If you come to Houston, I know the cove where they all float ashore)
* Inflatable Woman left over from friends Bachelor Party. This would be cool but kind of hard to explain under some circumstances.