Nice work! Love your editing, interesting content, good audio, etc. All systems go.
If I'm going to be picky, although I find most vlogs OVERuse music, I feel like some sparse sprinkling of good tunes could have spiced yours up a bit. I also would have liked to see/hear more reactions from your friend, but maybe they weren't too jazzed about being on camera?
For the thumbnail, I find red tends to be a bit over-used; when used sparingly or smartly it can be eye catching, but since it's overused it just tends to blend in. And since you had so many neat aquarium shots, I think a clean, simple underwater photo might work better. I appreciate your sense of humour, and I can tell you're trying to express that in your thumbnail, but I think the sense of exploration is the main draw and it works because your video work is quite good; I would consider emphasizing that more in your thumbanils.
Just my 2 cents. Best of luck with the channel!