Infact the Thames is fed by the North Sea which is at full salinity. So . . .
The Thames flow into the North Sea, not vice versa. Thus the association is described that the Thames flows into the North Sea. Honestly, have you ever heard anyone say that the Atlantic feeds the Amazon?
Plus, that area where the tide of the North Sea will affect water height and the salinity will be a mix of sea water coming in and fresh water flowing out. This mix of water which has a greater salinity than freshwater but lower than full marine is called brackish.
Ok, estuarine sea horses according to your link are the below. The scientific data does not state the SG that they are found in, however it does show that sea horses are found in estuarine areas, those where freshwater rivers flow into the seas and oceans.
H. abdominalis
H capensis (tolerates salinity of 1-59 parts per thousand, making it extremely euryhaline)
H. hippocampus
H. kuda (Noted as living in lower reaches of rivers and inhabiting brackish waters)
H. reidi
And those with the habitat unknown
H. algricus
H. fisheri
H. lichtensteinii
H. mohnikei
H. sindonis
So based on the above, your statement of:
There aren't even really any brackish seahorses
Appears to be contradicted by the scientific descriptions of two sea horses that can certainly inhabit brackish areas, another 3 which inhabit estuarine areas and 5 whose distribution is not currently known, and thus on whom it is difficult to pass judgement.