Find a buddy, and get in the OW, I am too a decent swimmer, but panic in OW... others don't, so without trying it's hard to say. One thing is for sure, the fear of the fear is already there and in essence I don't feel you're giving yourself a fair chance. I would very definitely try and get a swim in to "test the water" so to speak. Being a good swimmer and with the additional buoyancy that a wetsuit provides, combined with salt water, you won't fail physically only mentally in my opinion, but don't underestimate the impact that a mental block can have on physcial performance - i can swim for an hour or so straight in the pool covering 2500m - 3000m reasonably comfortably but still haven't managed 1500m (two laps of my local lake) without stopping for a breather and a bit of mental recomp before setting off for lap 2
I could say try a shorter distance, but being a good swimmer that will feel like a "cop out" i would imagine...
So
- try and get to the sea for a practice and see how you get on, you'll have a better idea of how doable that might be (don't forget the sea has currents etc which will have an impact on how the distance feels)
- if you can't do 1, i would drop the distance to the 800m but not the 400m, i would suggest that the 400m wouldn't really provide much of a challenge, even if you had a bit of panic (you could breastroke it, I would imagine!)
Please note i'm not a seasoned triathlete, but being an antipodean I grew up around open water and despite this now suffer with panic, I WOULD NOT give the same advice to an OW newbie normally, it's more to the fact that you have stated you are a strong swimmer and attend masters classes that i'm not treating you with kid gloves.
Above all mate, be safe and don't endanger yourself or others, swimming in OW is very very different to a pool.. there's no sides, no black lines, you can't see the bottom, you share the water with "things" other than fellow swimmers, you can't touch the bottom, it's dark and it's cold, there's currents etc etc etc...
Get a practive swim in ASAP but don't do it alone
Good luck and let us know how you get on