Tide doesn't really have serious weaknesses; he has mana issues for the first 20 minutes like almost every STR hero, but arcane boots on him and/or his teammates eliminate that problem entirely (also maybe a bracer or two and of course a wand.)
He starts with a decent amount of armor for a STR hero and has fairly middle of the road AGI for a STR hero. He has alright starting STR but very high STR gain meaning he quickly becomes very tanky even without items, and both Kraken Shell and Anchor Smash serve to further reduce the damage he takes.
His skills also continue to be useful at all stages of the game - Anchor Smash simply scales as expected, Kraken Shell becomes increasingly useful as Tide has more HP, and Gush will still always be useful for the -5 armor if nothing else.
I agree that Ravage can be trickier to use when the enemy team starts loading up on BKBs, but the enemy team just knowing you even have Ravage can make them use those BKBs earlier than they'd normally want to, shaving precious seconds off the effective time of the immunity (and one or two seconds is a big damn deal if you're down to a 5 sec BKB.) Tide's still useful even between Ravages because of Anchor Smash if nothing else, though I'll agree he's not nearly as big of a threat when Ravage is down.