Wearing a lycra seaskin under your shorty adds warmth and some protection against scrapes, jellyfish, and "boat bites!" Another nice thing is that you can use the long sleeves as a make-shift glove when needed. If there is current and I want to use the mooring line as a stabilizer during my safety stop, I just pull the lycra sleeve down to cover my hand - because the mooring lines are often overgrown with stinging corals and other minor hazards.
Once in Roatan they were having a bloom of something the locals called "Picu Picu". There were large rafts of thimble jellysish floating around and even though we avoided the jellyfish when picking a dive site, apparently the larva, invisible to the human eye, were abundant in the water and would superficially penetrate the human skin. They didn't cause permanent injury but people that were exposed before anyone realized what was happening, quickly erupted with an ugly red, itchy rash all over their exposed skin.
I saw a little local boy come running out of the water screaming during that episode and a guy from Germany was worried that they might not let him back into the country because he looked like he had a contagious disease.
The dive op quickly told us to cover up as much as possible and they provided a big jar of Vaseline to spread over any exposed skin, like the neck and face.
Ever since that experience, I have resisted the urge to wear minimal coverage during dives even in the warmest water. On our recent trip to Little Cayman, a woman on our dive boat only wore a swimsuit during her dives, even for the 3rd dive of the day. She said "I never feel cold in the water!" And I thought "well, you're lucky that way, but there are other things to worry about besides the cold" but I didn't say anything. The water was quite warm (85 F) and I just wore seaskins over my swimsuit, but I was happy to have them for both warmth and protection.