After several tests, I found out that the culprit is ELFX Enhancer. Skyrim has a degree of depth of field underwater which blurs stuff based on proximity, then there is yet another blur from what i've seen, which comes from the Enhancer option from ELFX (Enhanced Lights and FX) mod, making your character look terrible underwater, as well as many other textures. Disable ELFX Enhancer option and it should run normally again. P.S: Most probably necroposting but just since this is one of the first results google has for this problem, might as well make it known...
