The MK II Sea Hunter is easier to use in salt water and is more stable than the Infinium in those conditions, on the other hand I feel that the Infinium detects deeper but is a bit more difficult to use in salt water but only because you might have to fiddle with the discrimination control to get the detector to run quiet.
As far as the coils are concerned, the mono's are more sensitive than the DDs. The 8 inch mono is a beaut coil and is easy to manage in the water. The big mono will let you cover more ground but with only a marginal increase in depth over the 8 inch on up to largish coin size items, past that size the big mono will get significant better depth. The DDs tend to be more stable in the water than the monos but can get flipped about by wave action.
The 5 x 10 DD is a good beach coil but is not WP. This coil is less effected by elec interference than the monos and large DD and runs nice and quiet over the salt sand.
Just an opinion. Not Gospel.
Adrian SS