If boat builders adhered to the "petroleum storage regulations", you lot wouldn't be able to have motor powered boats.
A couple of years ago the owner of the fuel barge in Poole Harbour campaigned to have the carrying of fuel in a boat banned on the highways. His reasoning was that it's illegal to carry more than 10 litres or so in a portable tank/can!!!
If the tempo tank has the facility for a breather hose then connect it to a breather outlet, otherwise a vent of some sort should be fitted as low as possible to the seat pod.
Basically, if you have fumes, get rid of them!!
It is better to remain silent and be thought a fool, than to speak and remove all doubt!