I found an interesting ascii aquarium screensaver for Linux terminals called Asciiquarium.
I installed it tonight and took some screenshots:
A ship, castle and goldfish:
A large shark:
A sea monster:
Asciiquarium is a short Perl program that runs in a terminal. To download it, make sure that you have Term::Animation. On Ubuntu Linux, that means typing sudo cpan and then install Term::Animation.
The go to your downloaded Asciiquarium file (it's just a Perl program in text format) and chmod +x in a terminal to make it executable. Then type ./asciiquarium to start the program.