Found this with Google, I hope it helps!
Around 160kg is the empty weight, but then you may have to add fuel (at around 0.8kg per litre (say 20kg for 25ltr)). If you Google the weight of most engines you should find them (I'd guess around 100-150kgs for an "average" 50/60 hp engine) and I would guess a trailer for that size boat is around 250kg.
So if we say boat + 25ltr + 125kg engine + trailer = 160 + 20 + 125 + 250 = 555kg
as a starter for ten?
I'm sure you can get a better approximation if you know the exact trailer/engine/fuel tank capacity. Or you can do what I (stupidly) did and take it to the weighbridge!
But you can tow up to 750kg on an unbraked trailer, providing it stays below the maximum trailer weight for your Jimny (or maximum Gross Train Weight for Jimny plus trailer).
Hope this helps?