What Water to use?

[vermonter310]

I've been reading everything I can find on the net about Marine tanks in preperation of setting up my first SW tank 8O and I have a ton of question still. But the most pressing for me at the moment is , What water do I start with? I've read about the benifits of RO & DI filters and how they work, but do I need to use them ? They seem to be quite pricey, I live only minutes from the icy north atlantic should I use Actual SW? Tap water? What? Same thing for my TO water ?

Thanks

VT310