433Mhz transmitters/receivers that you can buy from China on eBay for $1-2
The very low cost transmitters are built to be garage door openers, wireless door bells and such. If the door does not open you can keep pressing the button.
XBee pro to get the distance you require
There are "pro" and non-pro versions. Also 2.4ghz and 900mhz. I have used 900mhz at great distances.
Water resonates at 2.4ghz and will block the transmission. I live in a very dry place. 30% humidity is high for us. Twice a year we get a fog and the 2.4ghz transmitters will not work outside. For years the governments did not regulate 2.4ghz because they believed no one would use that band. It works indoors for WiFi but not well out doors. Your transmitter will be sitting on the water and the humidity will be approaching 100% I think anything at 2.4ghz will fail.
XBee and the like are smart. They are transceivers. They will transmit the data and then ask if the data was received with out errors. If errors they will re transmit the data.
Some of these modules can work in "mesh" mode.
Example: PC---100m---senser 1---sensor 2---sensor 3---sensor 4 ----sensor 5 ---sensor 6 ........
Data can go from PC to sensor 1, then 1 can talk to 2, 3 and some times 4 but no more.
Sensor 3 can reach 5 but not 6. Sensor 5 can talk to 7.
When data must go from 7 to the PC it must be re-transmitted down the long line. (many short jumps)
Last time I did this project we used 900mhz XBee modules. To keep the power down we used solar and the modules were asleep most of the time. Every hour all the modules woke up and talked to their neighbors. Data was send and received. Then every one went to sleep. The power was on for 20 seconds every hour. The receiver was on for 1 minute while the transmitter was on for a very short time.