In some marinas, boats sit nicely in a gale. In my marina the current runs through faster than any marina I have ever seen...enough to pull floating docks edges under water and have breakers on them.
So that's half the question.
Ultimately dock lines rarely snap if even close to their working load...it's generally chafe that spells disaster.
If you could eliminate chafe, 1/2 inch would be fine...it's what I had the first 3 years...and may go back to it with loops at either end so chafe is only 1/2 as bad.
But it ultimately is what you like for handling, looks and chafe. Strength unless continuously stressed isn't usually the determining factor.
Only caveat, is if you spring in hard or really stink at docking...heavier lines could be warranted....