Pressure treated pine will last the longest if you maintain it well. I'd go to Dunn Lumber and check out what they have or can get you.
Then get someone who has a level 2 or 3 discount to buy it wink

There are different grades of pressure treating. buy a minimum of .40 pressure treatment. do'nt use the cheapo .25 grade. Buy 60 if you can find it and afford it for your project.

Depending on the type of wood you may need to pinch the boards together or gap them. Then seal with a good sealant after they have sat out and dried for 20-30 days.

Are you doing the whole dock surface?
_________________________
You know something bad is going to happen when you hear..."Hey, hold my beer and watch this"