Hello there,
I fitted plastic vet us tanks to a steel trawler I built, I still have them in and never had a problem with them, they came with no holes in so it's a bit of a hassle cleaning the cuttings completely when you drill them, apart from that they are very clean and if you go for the clear ones you can instantly see how much fuel you have.
Only problem I could foresee on a loch in is the fact they have few or no baffles so you will get some slosh when you give it the beans, I just made and fitted a pair of 110 gallon steel tanks in my loch in with 7 baffle plates in each and I get no slop
I think the main issue is how the fuel take off is configured. If your boat rides bows-up, no number of baffles will stop the stop the surface of the fuel from being horizontal. If the fuel take off is at the front of the tank, you could start sucking air when the is still plenty of fuel at the back.
I have put my fuel take off at the rear inboard position about 2 inches from the bottom to avoid drawing any water (god forbid) but have also put a balance pipe between both tanks for equalising the tanks, at the centre of the balance pipe I have put a ball valve to allow any water to be drained from the tanks without having to empty the tanks, hope this helps as I have learned from lots of repairs I have had to do for lots of boats over the years
