Jagex did JUST recently nerf this method, so we're still seeing how viable it is now before we officially host this event. However, if you wish to fish on another Ironmans boat, you need to have made the same tier of net on your boat at least once in order to use theirs. A high level boat will typically use a Cotton trawling net which requires 84 sailing and gathering 8 Ray barbs.
Once underway, we may catch whatever fish any host feels like catching. It is likely we catch Halibut (the new best bwan) or Bluefin (Prayer + HP) over Marlin because Marlin are really annoying to fish without a max boat.
10 players can be on the same boat at a time fishing.
Gain sailing and fishing xp.
You cannot leave someone elses boat without losing all the fish you have stored in their net. You cannot use their storage either. You have some options for banking:
Bring a knife (cut all fish into potion secondaries and fine fish offcuts)
Bank with Imp boxes- this is the best way to bank on an ironman. You can store infinite Imp Boxes in a Huntsmen's Kit. Imp boxes have 2 charges each and can send 1 item back to your bank per charge. This works with Fish Crates which store 10 fish at a time. Each imp box sends 20 fish back to your bank. Catch imps in the house basement in Kastori.
Fish crates- this is an alright version for routes that bank every lap or 2. You can withdraw fish and shove them in fish crates until your inventory is full. You may need to juggle dropping stuff to do this efficiently. It's viable but much more of a headache. Bring a knife to cut fish you don't have room for.
If someone captains for you, the way they bank is their choice. Come prepared with imp boxes if you care that much.