I have towed a horsebox compleate with two horses without any problem with my 2.5 Omega quite a few times
Don't horses ever get car sick? If you made me stand in a swaying box where I couln't see where I was going I'd chuck up all over the place! 
Thats probably why they dont.....coz they carnt see where they are going!
Thats what makes people seasick....they can see the horizon or the what evers out the window....which appears to stay still....but the movement of the boat isnt still.....so confusing messages to the brain.....eyes say still....body says moving.....result...not good.
If you do suffer with seasickness its best to find somewhere inside the boat with no windows!!
I however dont get seasick......and in rough weather....always make for the restaurant......its usually empty.....order a cooked breakfast....you usually get loads as they carnt get rid of it.....tho can be tricky sometimes stopping the plate sliding of the table