Water taxis won't take you to RAK.
£50 each way isn't so bad for a taxi. That's £12.50 per person per hour, which is way cheaper than anything you'd get in the UK (taxi or train, for that matter).
For comparison, it costs me about £50 to do the 1 1/2 hour journey between central Abu Dhabi and Dubai airport.
DXB to RAK is about 100 km, so a marginally shorter distance.