I have used two different WIFI based r/c devices:
1) the hobbyking 4 channel wifi receiver.
crap. it doesn't have a 70m range. more like 50m. it has no capabilities for FPV. it is "telecommand" only. I was constantly driving the boat out of wifi range. gave up on it after two outings.
2) Parrot AR Drone (original model) much better range, but still not adequate. probably 150m. has video connection to the drone. very limited functionality compared to "real" quadcopters.
The final issue both suffer from is the screen brightness of the controller device. a standard run of the mill android tablet will not have a bright enough screen to see anything in bright daylight. I used a 7" nexus tablet, which has a screen twice as bright as the average. for any FPV device that uses a tablet or phone for the remote camera view, this will be an issue.
in all cases of the WIFI setup, it is the tablet/phone that is the limiting factor. they are low power devices. they are not designed the way the 2.4ghz transmitters, or the 1.3/2.4/5.8ghz video equipment is designed. openlrs and comparable devices are reaching 20km ranges. best start would be to look at devices in hobbyking's fpv section
http://www.hobbyking.com/hobbyking/store/__543__172__FPV_Aerial_Video_Telemetry-FPV_Accessories.htmland figure out what you need to add to an existing boat to get FPV on that boat.