Get a "Spare Air" and a mask..
I keep a "spare air" 3.0 mini scuba tank w / attached regulator.. It is worth about 57 breaths at the surface.. (less than 33 feet/1atm) 57-60 breaths at 12 breaths/min = about 5 min of cutting..
Here is a link.. its about 180 bucks and can be kept in your boat.. Also good for checking under the boat for damage.. or going to shallow depths for dropped items.. (as long as it is clear water)
http://www.scuba.com/scuba-gear-33/Octos-Spare-Air.html
(I also use it for scuba escape/buddy tank in case of emergencies at depth)
A leatherman..
Trauma shears (the kind that cut a penny) with a strap
and a good sharp knife (dont forget wrist strap or you will be going to the bottom to get the knife.. )
I used the spare air once and it was perfect.. got the tubing rope in the prop.. the THICK one..