snails no, most fish no. there's only a handful of fish that are known jumps. if you have one of those using some eggcrate on top with do the trick (you can get it a home depot in the work shop lighting area).
As for covering the bulbs, HQI have to be covered because they give of harmful UV light, any fixture or retro fit will come will the glass you need. Single End bulbs do not need to be covered because they don't give off the UV. You should be keeping the bulbs probably 8-12 inches off the water anyways so unless you have fish that like to splash you'll be cool. You may get a little salt creep on the bulbs (i do) which is easy to clean of, i do it once a week take the bulbs off and wipe them down with some damp paper towel and dry them thoroughly. I also take this time to switch the bulbs as well. (don't think it's necessary but i do it just in case one bulb has more PAR then the other so each side gets each bulb for a week.)