Nothing is waterproof.
Some things are more waterproof than others :-).
Silicone rubbers are rather water permeable. What you are seeking to avoid is the presence of liquid water and air at the point where corrosion will occur. If you use a material with low water dissolved content and a tenacious voidless surface adhesion then you cannot get liquid water (no voids) and the dissolved water content is low so the reaction rate is low.
Many manufacturers make conformal coatings aimed at minimising water & oxygen related corrosion. Dow Corning is one such - better known than some and with a good reputation for quality products. I have no relationship with Dow Corning except as a customer.
Dow Corning Sylgard 184 is formulated specifically to meet the above requirements. It's main use if solar cell potting.
Dow Corning 1-2577 is a conformal coating which works reasonably well. It forms an ~= 0.1mm layer and can be sprayed brushed or dipped. Not breathing the fumes is a very good idea.
Dow Corming conformal coatings overviews:
Electroplastic - 1.2577 is in this group
Poormans CC: An initial layer of "polyurethane spray on clear coat varnish" may help. This sets by reacting with atmospheric water and is a low cost but reasonable conformal coating.
Clutching at straws: The presence of electrical potential in a corrosive environment will greatly accelerate corrosion. It may be that having the sensor powered is a significant contributor. If you are able to depower it for a significant portion of its cycle it MAY help.
EVA: I have not tried this but it has a moderately good chance of working. EVA is the traditional bonding and sealing agent of choice in Silicon on glass solar panels, which typically have lifetimes well in excess of 20 years. EVA goes a long way toward meeting the requirements mentioned above re water solubility, plus voidless adhesion to sealed surfaces. EVA is inserted as a plastic sheet between glass and silicon cells and then crosslinked under raised pressure and temperature. It seems likely [tm] that "just melting" EVA plastic over the sensor with a hot air gun has the prospect of meeting your needs. EVA plastic sheet is available from PV panel manufacturers or as high quality glasshouse (plastic house?) film . Laminating temperatures are usually in the mid 1xx range but low(er) temperature laminating EVA is available.