What brak says is 100% accurate. Most marinas, our sail club included, do not use even WEP wireless encryption (which is trivially breakable, anyway) because, well, sailors aren't going to want to deal with it.
As far as doing sensitive stuff on-line with your PC: Theoretically speaking: If the web connection is HTTPS (secure): That particular session
is as secure as if the wireless connection and everything in between was a dedicated direct connection from end-to-end. But, and it's a BIG
Estimates in the computer and network security industry are that up to 80% of private/residential PCs are compromised, and most of their owners don't know it. I'm an experienced Systems and Network Admin, and I'd
never consider using a Windows PC for banking, credit card or other on-line transactions. (I deloused one box at work that two AV programs and two anti-spybot programs, all top-rated, insisted
was clean. I found and removed not less than six (6) exploits, one of which was a key logger
Back to wireless...
On an unencrypted wireless connection: Everything you send/receive, that isn't itself encrypted, can be seen and logged by anybody else that's on that wireless network. And, since it's not secured, anybody, and I do mean anybody
can be on that wireless network.
I don't restrict my business customers from using whatever network access they can find, but our externally-accessible corporate email server allows only
encrypted, authenticated connections, whether they be web or email.
Btw: Another way you can help
defend against your Windows PC becoming totally owned is to not run with Administrator privileges. Create one account with Admin privs that is used only
for doing administrative tasks, and make sure all of the normal user accounts are "Limited." Then, once a month or so, boot the PC up into "safe mode," log on as "Administrator" and then
do all your security scans. You might luck out and not be compromised by one of the exploits that can get by even those measures.