Salt water is VERY UNKIND to precision metal machinery like rocket engines. Also the tank has such thin walls that chaotic wave action will destroy it.
Hm, this isn't intuitive to me. How could a rocket that was designed to withstand the pressures and conditions of space not be able to take some salt water? And what about just adding a layer of coating that would protect it?
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.