Hey mate. I use the esky scenario. not ideal, but it works, and you can get something of a watertight seal when moving to stop to much water splashing out. Yakkas are pretty hardy so consider the humble water cooler. you can get them on various sizes to suit the boat, and they're not expensive. a portable aerator will be enough to keep them alive, AND because they're insulated, it maintains the temp of the water really well. On top oif all that, they can be sat upon, locked down and fished around...not big tank to negotiate when hooked up!
You can plumb them easily to attach a bilge to supply fresh water to the tank, and have an outflow where the tap attachment is...
I'm putting on in my boat this week!
Cheers
Dan