I have used the Manfrotto Nano clamps for both windsurfing photos as well as regular photo work. It's rock solid and handy if you use it in combo with a very small ball head.

the problem with the Heady Duty Flex Arm is that it will swing.

Manfrotto makes a number of stiff extension arms with brackets that allow for different angles. Those extension arms then fit into the Super and Nano clamps. They are pretty sturdy and that's what you will find 100% of car photographers will use for taking photos off moving vehicles.

Stay away from anything Hama. In my experience their gear is made for casual consumer use not heavy duty wear.

With any of these attachments make sure you flush them well with fresh water after ocean use. Otherwise you can expect them to stop working very quickly.