Nervous...for first Tri this Saturday & ?'s

I'm doing my first Tri this Saturday in Bristol RI...Tri the Tri.

It is a Sprint Tri and I have been training since late April. My goal is to finish and I would like not to come in last, but finishing is more important. I have lost 75 pounds in a 1.5 years and was a big couch potato and now can run 3 miles very slowly...can swim 400 yards...hoping the other 100 can come with the race, and biking is not a problem.

My Big question:

Ocean Swim

I'm nervous, excited and a bit afraid of the ocean swim. I'm worried about the temp of the water and honestly it's the ocean. I was going to the location over the weekend to practice and check out the course but it was raining really hard. I hope to go this week before Saturday. Yesterday I spoke to a Tri friend and she told me about a Tri recently on the Cape and there were lots of jellyfish. I understand last year it was about 69 degrees. I don't have a wetsuit and have thought about getting one...but with 5 days to go is it nuts to change my plan. Promotion has sale ones in my size and compared to renting it isn't that much more for the type without sleeves. In NE does a full or sleeveless type of wetsuit make since? I hope to continue to lose weight so I don't want to get a wetsuit and not be able to use it next year.

My other questions are about my heart rate monitor.

My max heart rate for running is 185 and I have been trying to run longer and keep my HR in zone 3 130-148 whcih I struggle to do. I have done some running at the Y on a treadmill and find it hard to lower my heart rate even when I slow my pace way down. My running heart rate is usually in the 160's. When I was going slow and breathing easy but my heart rate was in the 160's someone I knew that had done a tri came and we talked for a few minutes. After she left my heart rate was 120, and then as I checked it my hr slowly rose to around 150. Any ideas why when I slow down my rate stays high even if I started running slowly the whole time?

When I play soccer and sprint for short times my heart rate goes up to 235 or so....does that occur for others of you when you do a quick but short hard sprint?

Thanks for any input and help....
Kathy from MA