Okay, so I didn't find anything about Medications, but I do know that babies can be in the water from birth with a few precautions. You might make sure to have some eye drops (just saline solution) in case her eyes dry out, but only use them as a salve, not a precaution.
Most simply, make sure she doesn't over-do it. There is something about water that tires you out faster than other activities might. The more tired she is, the less able to function she might become and the more dangerous things become (plus, if she is at all like mine, she'll get crabby).
In the salt water, you'll need to be careful about her drinking it and it getting in her eyes. For a while with new ones, when you blow in their face they automatically close their eyes and hold their breath. I'm not sure if that reflex is still in effect at 5 months. It would only take a quick check with her to find out. If this is still a response at 5 months, do this and then dunk her so she doesn't develop a fear of getting the water in her face. You can play, "I'm a little coo coo clock" you sing, "tick tock, tick tock, I'm a little coo coo clock. Now I'm striking one o clock". Then blow and dunk once. Start over again but this time, strik two o clock, dunk twice etc.
Don't forget the sunscreen for the places not covered by what I imagine must be a rash guard suit from what you described in your question (and it needs about 30 minutes of time between application and the water to real develop the shield on the skin). Also have her wear a hat that can get sandy, salty and wet to help protect those eyes. If she's pretty easy going you might want to get her goggles (they do sell them in very small sizes for toddlers and with uva and uvb protection in the lenses as well - these would help with keeping the sun and water out of her eyes). With the sun reflecting off the water its effect becomes much stronger. Eyes can get sun burn rather quickly too, if goggles won't work because they are too uncomfortable then you can also get her a pair of sunglasses. Check the sunshield rating, many sunglasses don't actually provide much protection - this one is definitely worth the extra expense.
Here is a pretty relevant article about 5 things to think about around the water with kids. considerations specifically related to safety around the water with kids. It only takes a minute and an inch. She suggests floaties, but I hear floaties are actually NOT a good idea because if the child's arms get tired the floaties don't ensure the face stays out of the water so a life vest for her size is probably your safest bet. (they can also slip out of inner-tubes so those may be fun, but not really a safety device).
Bring along some "splash toys" like a baster from your kitchen. Here is an article about great things for water play for your little one, to help her splash and play while she sits in the shallows with you without a whole bunch of extra cash going into it.