I just found a rescue-report about two kayakers in the UK who used an SMS as medium to ask for help. Although there are better ways, it worked and they were rescued.
However, as you can read below, the Coastguard made comments on that.

Maritime & Coastguard Agency (http://www.mcga.gov.uk)
Press Notice No: Duty Press Notice
Saturday, October 27, 2007
Posted 16:26 GMT

TEXT MESSAGE LAUNCHES SEARCH FOR CANOEISTS

At quarter past one this afternoon Clyde Coastguard were alerted to two canoeists in trouble in the waters of Loch Shiel after one of the canoeists sent a text message to the owner of the canoe hire on his mobile phone.

Clyde Coastguard immediately tasked Coastguard Rescue Helicopter 100 and Fort William and Mallaig Coastguard Rescue Teams to the scene. The Coastguard Rescue Teams proceeded to Glenfinnan, however with no direct road to Glenaladale where the casualties were located they had to use a boat from a nearby fishing farm to reach the scene.

The two canoeists were located shortly afterwards by the Coastguard Rescue Teams, and were then winched to safety by the Coastguard helicopter. The canoeists were taken to the nearby Glenfinnan Hotel where they are reported safe and well and did not require medical assistance.

Tarik Yassin, Clyde Coastguard Watch Manager said:

We were informed of two canoeists in the waters of Loch Shiel today after one of the casualties contacted the canoe hire via a text message. We overcame the difficult terrain and remoteness of the area to perform a successful rescue and are happy to report that the canoeists are now both safe and well.

We would recommend however that all personal watercraft users carry a VHF radio if they are going out onto the water, as this will allow you to contact the Coastguard directly and your signal can be tracked to inform us of your location. Although it is sensible to carry a mobile phone as backup to this we would always recommend that people dial 999 and ask for the Coastguard if they find themselves in trouble.

Posted By: Rosie Tapping

For further details contact:
The Maritime & Coastguard Agency Press Office
023 8032 9401