3yr old past 2 times in saltwater, sun, sunscreen gets red splottchy puffy rash under eyes and on cheek. Puked this time. Slightly sun burnt on back.

Allergic reaction or sun exposu. This sounds like a generalized allergic reaction. It could be to salt water, a pathogen in the saltwater, the sunscreen, or something in the sand. Also combination of sun exposure can cause dehydration which can present that these symptoms. Push fluids and administer benadryl (diphenhydramine). If in doubt take the child to the er or pediatrician.