Yes, but only to a degree. Adding water to a free flow mattress will make it firmer, and removing water from a free flow mattress will make it softer. However, the converse is true for semi-waveless and waveless mattresses. The more water added to these, the softer they will be, and the less water added to them, the firmer. It is important to note that free-flow waterbed mattresses do not get very firm, so your best option is to opt for a waveless mattress if you need more support for your back. This type of mattress can get firm, and some are designed with lumbar supports.