I've done the reverse and have started using 'freshwater' tackle and rigs in the sea. These days fish in the sea seem to be getting smaller and fewer and therefore because they have more water to swim in are considerably more difficult to catch than their cousins in a commercial fishery. (Leading Statement N0.304)
I use a 17' commercial fishery float rod (that's what it said in the advert!) and use it for snatching little 'uns (pollack, wrasse and pouting) Great fun and fascinating to see what bait is catching. I choose a deep water rocky mark and chuck in some groundbait (usual stuff from the fishing shop inland mixed with ultrabite and sardine/pilchard oil and chopped up frozen sandeels) The rigs are straight out of 'Improve your Coarse Fishing' and John Wilson's book.
Recently I bought a Shimano Hyperloop heavy feeder (it says it will cast up to 150gms! that's about 5oz!) Off clear beaches I use a method feeder with the above mixture and a 30cm hooklink and size 6 and smaller hooks. Casting short just over the breakers I've ben connecting with pouting and whiting about the same size as those I was catching much further out on the beachcaster! You pays your money etc. etc. I use braid on both rods - helps in the wind and current and the soft action of both rods cushions big fish bites (This is, so far at least, theoretical!!!!!!!!!)
I bought a book on 'Fishing Light in the Sea' and it had pictures of a Pole being used so I think that anything goes. The main difference between Sea and Fresh water usage is the God of Fishing - CONVENTION! (Leading Statement 305) (No one uses tackle like that here!)
The obvious differences are of course currents (you try holding bottom without weights developed to do just that), snags (largely unseen and difficult to avoid because that's where the fish's food lives) In other words there are places and times (winds and tides) where it would be totally inappropriate to use light coarse tackle SO DON'T! Look for suitable places and tides (I now enjoy neap tides for example - things move slower and stay the same for longer!) I bet that a REALLY good specimen hunter would master sea conditions straightaway.
Oh, you may have crayfish in some places in rivers, but the bloody crabs play havoc with light tackle in terms of bites, you keep thinking its a slow take-away bite like you sometimes get with flounders and eels in slow moving shallowish water. Otherwise try freelining with sandeel, King Rag or a nice Peeler. You won't need leads and if you sang up all you lose is a hook.
PS I'm experimenting with Boilies soaked in fish attractant, but that's a secret!