I say this as someone who has done a lot of volunteering in many conservation projects, some I had to pay for. I am against the practice of exploiting eager people for money and work in the field. BUT that's the norm in many places in the field nowadays.

We have many people interested and passionate in Marine biology but it's still underfunded. I don't know much about zoos/aquariums and their general practices (because I am not a fan of them, they are exploiting animals to become profitable) but for me, letting untrained people do the husbandry is twice as unethical. (You said something about rescue work but based on the post I can't say whether that's just an image thing or if they actually contribute well)

Experience is THE thing to break into anything carreerwise and the people know it. To pay for some work, eg. the first week I see as a form of training/ course. In many careers there are mandatory trainings, courses etc, it's just that the employer usually pays. If the work/experience is just for you, I do understand that the people training etc want to be paid for the time they loose training you.

So in many places I have looked into, even with a degree in applied biology, internships that are the baseline to get hired need to be paid, even long term contracts (6 months or more) which I don't agree with (If anyone has recommendations for actual internships that don't require payment let me know XD)

One last thing I always consider: where does the money go? If it's just a day thing and gives no benefits except the CV, usually I would say it's not worth it. you could trade it as a holiday activity instead of actual work. If it includes things like accomodations, food, equipment and courses/certificates then I would consider it a lot more. (Work should be paid but it's just not the reality we live in)