Mmmmnice,
RN helos tend to use Prudent Limit of Endurance (PLE), which is the point you reach MLA. If you bust MLA you should then be declaring a PAN. This can be made more restrictive for more inexperienced crews/poor weather etc by imposing an additional RAMP fuel on top of MLA.
There's also MIFF (Minimum Indicated Fuel for Flight) which if you go below, it will shortly get very quiet up top, although we don't quote a "time to MIFF".
"Time to Divert" lets the ATC guys know how long they can mess you around in the circuit before you're going to b*gger off and land somewhere else.
The only difference between land or sea ops (besides getting wet) is when overdue action would be instigated - considerably sooner if you're over the sea.