Throughout pregnancy it is important to avoid overheating and dehydration. Pregnant or not it's crucial to protect your skin from the sun's damaging rays.
But you don't need to hibernate inside throughout your pregnancy. Go ahead and enjoy the outdoors while heeding a few precautions. Whether you are lying out in the sun or outdoors exercising it is important to avoid becoming overheated and dehydrated. So, I would recommend avoiding being out in the heat for extended periods of time during the hottest part of the day. Try to enjoy outdoor activities and exercise in the morning and evening as much as possible. While you are out in the heat, sun or no sun, be sure to drink plenty of water to stay hydrated. And of course you are right to use plenty of sun block, at least SPF 45, reapplying every few hours and after swimming or towel drying.
So, this trip may not be the best type of vacation for you right now if you are planning to spend most of the time out in the sun during the peak hours of the day, but you may be able to enjoy it if you are able to stay shaded with a cabana or a beach umbrella and able to drink enough water to stay hydrated. If you are not ready to share your news yet, you can simply tell your friends that you are serious about preserving your beautiful skin from sun exposure.