Offsite catering on an Island
Given as you are in North Bay, Ontario - you'll need to follow the Ministry of Labour rules on paying your staff so your starting point has to be to pay them the hours that they are actually working - that's a given. There is no requirement to pay them more than that and for 30 hours there is no overtime payment required unless they somehow accumulate more than 44 hours work in a week.
Given as you have an island venue you could determine that it is a remote or camp location - I don't mean remote as if going north to the diamond mines etc. but remote in that it is is not readily accessible and remote/camp work does generally carry some sweeteners - as you do want to incentivise them to go and do a good job for you; so some extras you might consider are:
Pay for travel time - i.e. if you are paying $11.25 an hour for the working hours, pay $16.25 for the travel hours. Use Google maps to determine how many hours are to be spent travelling, not forgetting the time on the boat.
Throw in a Timmies/refreshment stop to/from the site.
Paying for the accommodation and their on-site food (although some camp situations do say take your own food) would be reasonable - without knowing your island location it is hard to say but an alternative would be a per diem to stay at a pre-booked motel - so that you know where to find them.
I come at this from a time served (also Northern Ontario) HR professional and have sent folk into remote/camp work situations. I would not 'feel out your staff' as Meezenplaz suggested but rather put together an attractive package rather than have the staff ask for more than you can deliver. Chefbuba suggests fattening their tip, if tips are an option that could work, another alternative would be a completion bonus.
Hope this helps, I'm new to the site so I hope this isn't too late.