(and I imagine that it is)your brother can have an ordinary BT landline installed - hundreds of houseboats have them. Provided the local exchange is ADSL enabled he could even have a broadband connection.
If for some reason a landline isn't a possibility then in theory you could set up a broadband router in the office and distribute the service to the various houseboats via an ethernet network. Ordinary CAT5 cable is good for distances of up to 90 metres, but you must be very careful to keep it clear of interference sources - anything that generates a strong electro-magnetic field. You would also need to protect it from the weather, and from abrasion. The best way to do this is to run it inside black plastic electrical conduit. Alternatively you can use the more expensive Shielded Twisted Pair (STP) cable, which is stronger and bulkier. You would still need to protect this cable, but you don't have to worry so much about interference.
Each computer would need a Network Interface card (NIC), but otherwise there should be no technical problems - the Internet connection would run on the router, so you wouldn't have to run a computer in the office. If ADSL was available the office phone would still function as a phone. If you do this I recommend that someone reboots the router at least once every 24 hours, so that the connection doesn't run on a fixed IP address all the time.