Yes.
are tides useful?
Tides are useful because they can be used as a source of renewable energy (a source of energy which will never run out). The energy of the waves can be harnessed to produce electricity, and no carbon dioxide emissions are produced.
Tides are also imopiortant to the life cycle of some marine life.
Yes! Without them some marine life wouldn't be able to thrive, better yet, survive. As Anna said they can also act as a source of renewable energy.