I didn't get a fishing rod before, now I cant go to Dewford anymore and I don't have any water Pokemon to teach surf to and I have no idea what to do... I'm completely stuck.
Zigzagoon and Linoone are able to learn Surf, among other Pokemon, so your best bet would likely be to head over to one of the early-game routes, catch a Zigzagoon, and use that to make your way over to Dewford.
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Hope this helped!