Most people use hammocks the "wrong" way.
Hammocks were developed in Mexico
where they lay on them angled across the hammock NOT lengthways as the North Americans use them.
Hammocks used properly do not hurt your back and are actually recommended by doctors for back problems.
The mexican "string" hammocks are the best, by far, but are more expensive.
However you must lay across them.
I agree that they really are not suitable for use underway.
I used to own a company Australian Hammocks Pty Ltd
How to use a hammock