I remember making "wave machines" in plastic bottles when I was younger.
You could say this is a more romantic version.
It's fairly easy to make. The hardest part is finding the tiny bottles.
I used glass bottles but you could use plastic, which can really float. If the small bottle can float you could skip the string step. Just make sure to weight the small bottle with enough sand to float at the right level.
Step 1: Gather Materials
2 bottles - 1 small enough to fit inside the other; labels removed, washed and dried. (I used a small sample of Tabasco for the small and a sea salt bottle for the large.)
Baby oil - enough for 2/3 of your large size bottle
Small amount of sand - to go inside your small bottle
Water - enough for 1/3 of your large bottle
Blue food coloring - or whatever color you want
Glue - appropriate for glass or your materials
Nylon craft string
Spouted Measuring Cup or pitcher
Small paper and pen