If you get a window seat, offer it to the guy on the isle seat. You're gonna need to get up to pee/stretch, and often that means waking the guy on the inside isle seat up. I made this mistake on the way to and from mexico in October when I was about 18 weeks.
If you're going to an all inclusive resort, order your drinks directly from the bar, make sure you clearly state you want NON ALCOHOLIC and WATCH them make it. Question what they put in your drink if you need to, and get rid of it if you are worried. While in Mexico, I pretty much just ordered "something with no alcohol" all the time. Usually it was fine, but some of the bartenders were ignorant a-holes and looked at me like I had 3 heads. I didn't look obviously pregnant, but whats it to them if you don't wanna drink??? At one point, I was in the lobby bar, and asked for something non alcoholic. There was another resort worker sitting at the bar across from me. I WATCHED the bartender grab a bottle of rum and make like he was gonna dump some into my drink in front of this other guy. The other guy gave him a look and shook his head, so he put the rum down, but holy crap. A-hole. I didn't feel the need to explain to every single bartender that I was pregnant. This all being said, my sister in law went to mexico at 20 weeks pregnant just after we did, at a totally different resort, and said she had absolutely no problems at all. Just be careful :)
Drink lots of water and have fun!