Your definition of STRONG fishman is whats lacking.
Jinbei is not STRONG, he is a Shishibukai… Which are suposed to be (according to the title) the 7 strongest besides the Yonkou.
Obviously thats not true. But you couldn't expect another fishman at Jinbei's strength, it would just be awkward. Jinbei is like, damn STRONG. We got an end arc based on HYPING is strength. He could easily match Luffy's gear 3rd feats booth post, and after timeskip. And that is ABOVE water (Well, we only saw Luffy fighting underwater after the timeskip so we can't compare it too much, and thats actually whats holding me from considering Jinbei a part of the crew. We don't know if he is, or isn't stronger than Luffy.)
And other than Jinbei, the princes are pretty strong. They were capable of dealing with that threat, if not for the E.S. :/.
The point is not THIS ISLAND PEEPS ARE WEAK, THEY SUCK.
Its simply, STRAW HAT CREW IS OVER 9K.
No mere police forces, or national army can dare to hold them now, only capable forces from Marine Headquarters, and thats how it is.
I think your problem is that 'Fishman > Human'... But what logic would it have been for a super strong fishman to appear? If there was one wouldn't he have joined the war 2 years ago. And if it was one of those enemies douchbag, wouldn't it be weird for the crew to go all out from the start. It would just be cheap.
I think the only way to make the crew strugle now, is with a KNOWN threat. Like Dofla, or a Younkou crew member showing up. But never with another strong character appearing out of Oda's ass. And which douchebag with power to swim up, would remain a frog in the bottom of a well? Another strong Fishman CAN exist, but him being in Fishman Island is not even logical. Why would he be there. If he was with the King, he would have gone and help with the war (Plot point could be made that he HAD to stay and protect, but considering WB was the main source of protection, he would be really conceited to think he would matter at all). If he was with the fishman radicals, they would have gained control over the kingdom as soon as WB went down. Jinbei wasn't there, it would be as easy as taking lolipop from children.
With that, any strong fishman will be in a pirate crew which DOES NOT sympathise with WB, or was to far a way to do something.