If you are heading out over open water many people will tell you take a twin engine plane. But that may be impractical or not possible if you are not rated to do so. You may want to consider investing in or renting (if you can) an EPIRB if you are worried about the open water section. Some type II PFDs are cheap and may provide a piece of mind. Depending on where you are the north atlantic is very cold so ditching is more of a time issue than anything. No need to go crazy with it and useful load may prevent it but you can always get a life raft.
As listed here the area can see thick fog roll in from the ocean. This will pose 2 issues for you, one is substantial worse than the other. On the safe side you may be stuck on the ground when you need to get the rented plane back. This may be no more than a monetary issue (if they charge you some late fee or something). The far worse case is being stuck in the air and having to divert (potentially far) to land safely under VFR. Keep in mind that even under IFR there are minimums and ground fog may knock out even an instrument approach. Either way it still gives you a solid other option.
You should really get to know the emergency procedures for open water ditching and rescue as per my earlier notes as well.
I assume you will plan a fuel stop somewhere in Maine (right before you turn out over water). That is a leg I would want to tackle with full tanks any way. Keep in mind 3.5-5 hours in an archer or 172 can be a grueling task. There are lots of fatiguing factors here. I would try and get the plane and just fly in your area for 3.5 hours (if you have not already) to make sure you feel comfortable being up for that long.
If you use anything digital like an iPad etc, backup batteries are a must. I would even being back up paper charts for this journey. GPS could be a life saver too.
I would think customs is some how involved but I have never flown out of the country so I don't know, if you have not you should look into it and make sure there is nothing you need to do ahead of time.