It's surprisingly simple! I designed an SVG quick in Sketch, and did it in a way where I can shift things left to right. Then a few CSS animations just move things left to right. When rotated, it looks like it's rocking!

Here's the initial SVG: https://p195.p4.n0.cdn.getcloudapp.com/items/OAuqBqvx/boat.s...

Everything else was just CSS