Dear Rob
what you say that i understand...but in Naval field still companies for quick results depends over potential based solver to produce Wave resistance.
I have used in past potential based solver in University , there we can also show wave elevation along the length of hull........so if by chance i want use Inviscid model, than still i can use Multiphase??

Can you also tell for external flow simulation, like of my case with big fluid domain (500 meter X 80 meter X 180 meter), what will be minimum mesh size value in general??...if i put 50 mm for volume target mesh...will it be too small?
As i am getting 33 million overall cells in my fluid domain, which i doubt is too much, because i used, i think very small size(like 20 mm)..Can you please confirm? is it too small?

Also like in software like fine Marine, we can have grid based refinement, which use to be easy in case of mesh convergence study.....here in fluent to do mesh convergence, which mesh size should i make finer every time??I guess i should refine BOI(Body of Influence area), which is covering hull and free surface?Please can you tell.

And if you can also tell what is this warning telling about..i cant get it