This looks like an error in the service, not in the ability of QGIS to access the service.
https://wcs.hycom.org/thredds/wcs/GLBy0.08/latest?service=WCS&version=1.0.0&request=DescribeCoverage&CoverageId=water_u_bottom&https://wcs.hycom.org/thredds/wcs/GLBy0.08/latest?service=WCS&version=1.0.0&request=DescribeCoverage&Coverage=water_u_bottom&
A describe coverage request on the water_u_bottom coverage shows an invalid CRS (EPSG:0 [Latitude_Longitude]):
<gml:RectifiedGrid srsName="EPSG:0 [Latitude_Longitude]" dimension="3">
<gml:limits>
<gml:GridEnvelope>
<gml:low>0 0 0</gml:low>
<gml:high>4499 4250 39</gml:high>
</gml:GridEnvelope>
</gml:limits>
<gml:axisName>x</gml:axisName>
<gml:axisName>y</gml:axisName>
<gml:axisName>z</gml:axisName>
<gml:origin>
<gml:pos>0.0 -80.0 0.0</gml:pos>
</gml:origin>
<gml:offsetVector>0.08000000976779562 0.0 0.0</gml:offsetVector>
<gml:offsetVector>0.0 0.04 0.0</gml:offsetVector>
<gml:offsetVector>0.0 0.0 128.2051282051282</gml:offsetVector>
</gml:RectifiedGrid>
Note that the reference to urn:ogc:def:crs:OGC:1.3:CRS84 here is a reference to the description of the envelope, not to the CRS.
When you try to load a coverage, QGIS uses a default CRS (OGC:CRS84) to give a GetCoverage request like:
But the server replies with an HTTP 400.
You can access the service in QGIS if you use a full GetCoverage request without any CRS, as a raster layer specifying an HTTP/S connection instead of a file, like: