Although, I could've sworn Mr. Briney refused to let you go to Slateport until you had the Knuckle Badge.
No, all you need to do is deliver the letter to Steven. Of course, unless you've memorized the layout of Dewford Cave (again, played way too much HOENN), you need Flash (and thus the second badge) to actually deliver it.
This is literally what I mean by hoenn's that awesome.
And actually...you don't need flash at all in that cave, it sure helps, as it massively increases the viewing radius but the smaller unflashed one is still easy to use, you need minor pathing memory yeah, but not a whole lot.
I often would bypass Brawly and go on to Slateport's beach to gain some extra levels, especially when running a Lombre, which really needs those levels to make it's absorb worth the upcoming-at-this-point rival battle.
Then I go back to Brawly and he's even more of a shove-over than before.
then Wattson gets discharged and then if i seriously don't have a Lombre on me, my rival battle is nigh impossible because i always Run Blaziken May as my hoenn playable.
That or Swapmert Brendan, in which case i have to push HARD for Ice Beam TM.
Note that a lot of my GEN3 hoenn excursions were in the days that GEN4 was around...so trading was not an option.
EDIT:
Just wanna throw this in there, but I remember reading in this thread a few pages ago the new Brendan and Magma designs weren't too popular, Personally i like them but i agree on those shoes on Brendan, also May's are the same way but her artwork's angle makes them less...blechy. however they look like heavy-duty water shoes, given hoenn's hugely vast wet forest environment, it's a good thing they were given such...
I really do hope character custom is available in ORAS, so we can still personalize our trainer so we're not locked to those shoes and etc...but iseriously hope they don't lock any outfit pieces to multiplayer rankings...that'll annoy the ******** outta me!