This depends on the type of internship you are looking for.
Paid interships are very hard to find, exept maybe for interships covered by exchange organisations like Fulbright in the US or GAES/DAAD in Germany. Anyways, I think it is almost impossible to get these now for this summer as you have to plan them more in advance. I am not sure whether you could get an Erasmus stipend for an internship but this also needs some preperation time before.
If you can do an unpaid intership, it is quite easy just to get that as you are of (almost) no cost for any group then. You then can just write to group leaders with topics you are interested in or try to get in contact with PhD students in that area. For example, I am just doing an Internship where I met a PhD student at a Bioinformatics Unseminar and asked him what he does...
A third option are obviously summer schools (and camps) which you can find (for the EU) on various websites. Some of them are actually quiet cheap or even free and allow for a very similar experieces like an intership.
I hope that helps a bit (though it is not about marine biology)