Many anglers live in cities or miles away from open water. You are right to get the most out of a saltwater fishing trip you need to be able to place your fly accurately near your target fish. When asked I advise them to go to a public park and practice casting to targets placed on the ground at different distances and locations. This is meant to simulate the guide pointing out tailing fish. I live in London, England. I have a dog so I use his rubber rings as targets.
Yes you will look silly. You may even get some daft comments but who cares. So long as you improve your skill so you can really enjoy your saltwater fishing vacation.