We are from Canada and I am in Mexico right now with my 10 week old. Where in Mexico are you going?
Don't drink tap water! I've never stayed at an all inclusive resort, but I imagine they will supply you with bottled water, use that to make your formula. I bath my DD with the tap water, but
I wouldn't allow bath toys that your LO might put in his mouth during the bath. I take my girl swimming in the pool and there has been no issues.
Family is extremely important to Mexicans, and so they LOVE babies! Everyone (including men) will want to say hi and possibly hold your baby. They'll want to know the name, age, etc. it would be a good idea to decide how comfortable you are with a stranger asking to hold your baby and come up with what your reply might be (and learn how to say it in Spanish). I let people hold Isla all the time and haven't had any problems, they genuinely just want to cuddle your baby. I've never seen men get so excited over a baby before!
On the beach the vendors walk up and down selling souvenirs, some of them sell stuff for kids. If they see you with a kid they will keep coming back to try to sell you things. Just say 'No gracias' and then don't look at them. Any eye contact and they will keep trying to make the sale.
Where we are (Cabo San Lucas) the sidewalks are not very stroller friendly, so a carrier might be a better option, but they can get very hot. We have both and then decide what to use based on the weather and what we are going out to do.
In the mornings go stake your claim at the pool (put your towels on the lounge chairs) so you can get a spot with an umbrella so your LO has shade. If all the umbrellas are taken, ask the resort staff and I'm sure they will get another one for you.
Be prepared for you LO to feed more at night than usual. It's hot and fluids need to be replaced. At home my DD wakes twice at night to eat, in Mexico she has been waking 4-5 times. Also take both warm and cool sleepwear for your LO. Some places keep the air conditioning cranked do it's freezing inside, other places have crappy AC. Ours is crappy so my DD has been sleeping in just a diaper.
Be prepared for you LO to be fussy and to sleep a lot the first few days while he gets used to the heat. Isla did nothing but cry and sleep the first two days, then she was perfectly fine. Yesterday we spent 6 hours on the beach and there were no tears.
That's all the advice I can think of for now!
