There can be many reasons for volunteering in the marine conservation/research sector and there are equally as many opportunities and providers. It is therefore worth looking around to find the opportunity that fits your needs. From an eco-holiday to serious CV building, from free to considerably expensive and from ad-hoc volunteering to extended durations overseas it is worth noting that there are some providers at all levels that are either exploitative or serve no conservation/scientific purpose.
To help get you started I have provided a list below of organisations and projects that have a proven record in delivering scientific/conservation results or come highly recommended as training opportunities.
For those of you currently studying at university one of the best ways to gain experience is to volunteer in your department. Researchers often need an extra pair of hands. If you decide to do this please be aware of your availability and how much you can commit. It is much better to commit to a few hours every two weeks and stick to it than to promise a full day every week and let people down.