Personally I think the most important thing on a boat
, where grounding can be unreliable (it shouldn't be, but frequently is) is the GFI/RCD. Since you say you have GFI outlets you have that covered.
In the US, if the appliance (new ones) is UL listed and has a two-prong plug
then it must be double-insulated, otherwise it must have a three-prong plug
. I'm quite comfortable with two-prong + GFI (I put more trust in the GFI than good PE ground on most boats).
If you're talking about old/used appliances
then I'd want to confirm the construction, but anything even remotely current
(20 years?) that is UL listed and with an original two-prong plug is double insulated.
That's an opinion, I'm sure you'll get the opposite from someone shortly.