Same boat as you when i started. Entered a OW std dist tri and could only swim about 100m before i had to take a rest. I think i did 7 swim sessions before the big day and was bricking it. I built up by doing sets of 50m and then resting 30 seconds. 25 sets, so 50 lengths total, then 30 sets and so on. Once i was doing some volume, the swim fitness built enabled me to swim further without resting. Managed the 1500m swim without resting, so it just needs some solid work. Agree lessons will help as well.
Regarding whether you will be faster or slower than in a pool. I don't think OW swims in tris are measured that accurately (or measured at all). I have done the last two Worthing tris and i was quicker on the first, despite being faster in the pool around the time of the second. Most were slower in 2014 than 2013. At a seperate oly tri, the billed 1500 turned out to be about 1850. I reckon organisers just put the bouys in and think yep, that's about it. Or do they canoe round several times with gps and really try and make it spot on??
I did Outlaw last year and ran 10 miles on the saturday afterwards and a 40 mile fastish club ride the next day. Felt really good. If you've been training/racing for years, reckon you'll be fine. If you're a newbie and have a newly built base, give it a miss and concentrate on the Outlaw.