Good hygiene is a highly subjective benchmark. Clean enough for comfort might be different than clean enough for company.
Ocean water, albeit salty, is water. You can bathe with it, wash clothes with it, wash dishes with it etc. Soap behaves differently (the surfectants don't suds as well) but you'll still get things cleaner than they were.
As for bathing, the obvious downside is you will have salt on your skin when you dry. For some this isn't an issue. For others this can be an irritant.
As far as polluting the water goes: dilution is the solution to pollution. The ocean is vast. You washing your clothes/dishes in it as an individual camper / kayaker is not going to matter. (Though a city dumping their waste water into it -- that matters.)
To address some of your suggestions:
brush your teeth with sand
Great idea, if you don't like having enamel on your teeth. If you have a toothbrush, and no toothpaste, why not just use the brush (with salt water, or dry, up to you) to scrub your teeth. I can't find the article at the moment, but have read that toothpaste does very little beyond giving you minty breath -- it is the mechanical action of the toothbrush itself that removes the buildup on your teeth.
take 1h sun baths
UV does kill stuff - but although it may kill some bacteria, it won't remove the oils, dirt, dead skin cells, klingons, toe butter, and mank collecting on your bod. UV also kills skin cells. Though I guess if you are in the sun for an hour, the resultant peeling sunburn will sluff off some of the funk.
let the clothes take 2h sun baths
This will kill some of the bacteria growing on your clothes, but same as above, it won't remove the oils, dead skin cells, dirt, etc that has been impregnating your clothes.
Not sure what you mean by "without budget." Good hygiene doesn't cost much, but bad hygiene can cost you comfort, health and friends.