According to the spec sheet, the device itself is intended for indoor use only and is not weather proof.
So, would have to be mounted in something like a waterproof dock box. Ask them if the sensor cable can be spliced, or ordered with a much longer cable.
An alternative would be a floating pool/spa remote thermometer, wireless, but not WiFi, uses its own indoor display...most of these devices operate on the 433mHz band, like garage openers, car keyless remotes, etc.
That particular one only has a transmit range of 100', but others on the market claim 300' range.
The LilyPad, by Poolsmart, is WiFi, but distribution in the US seems to be on hold...it's a Swiss company.
AMZN previously listed it, but now shows the product as unavailable, yet the free phone app continues to be downloadable.
How difficult would it be to trench and run cable to the house? That would expand your choices, since there are even temp. probes available for multimeters.
Also, you may be able to find a web site that shows current water temp. for your particular body of water, the cheapest solution. I'm on a 130-mile long lake, and there are several gaging stations with websites...surface water temp. is nearly the same everywhere on the lake, but we have thermoclines that can cause a 30° variation with depth, which varies tremendously on location.