By all means use tinned, marine grade wire. There are lots of places
to buy it on the Internet... don't waste your money at West Marine et
al. Get good tools like a ratcheting double crimper and use good,
nylon terminals. Here are some of my favorite places to buy this kind
of stuff. No commercial connection, just a happy customer.
http://www.terminaltown.com/ great place for terminals and tools
Wire sources:
http://www.bestboatwire.com/
http://www.jamestowndistributors.com/
http://www.pacermarine.com/wirecable.html
http://stores.ebay.com/Genuinedealz

Here's a little freeware program that will help you figure out what
size wire you should be using: http://www.midcoast.com/~aft/index2.html