The Umi no Tamagotchi is one of the hardest I've ever played with. It was a lot of fun, fast moving, and grew relatively quickly compared with other vintage models. However, in return for that, it was also one of the most demanding I've ever played with.
If you're someone like me who prefers to hatch in the mornings so they'll change before you have to go to class or work and not pause your Tamagotchis since it then throws off that growth, you're not going to be sleeping late or taking any naps. You probably won't be leaving this little guy alone much at all.
It loses happiness and grows hungry very quickly compared to previous versions. Once it finally hits the teen stage, it grows a little less needy, but you'll still probably find yourself needing to feed and play with it every 20 minutes or so. And if you're unlucky enough to find an octopus during your game, that can easily turn into a long cycle of game-playing to raise its happiness.
You'll find yourself having to constantly monitoring for Polar Bear attacks, since even one attack can seriously injure or even kill your little pet.
I was lucky enough to get one of the best-care characters on my first try. He's a little frog-Tamagotchi, the same kind that you can see on the Mori no Tamagotchi sometimes. I feel a little bad about that, but I think his cuteness helps make up for it.
I didn't have him as an adult for long before I had to remove the batteries. It was simply too demanding for my current schedule, at least while trying to also run multiple other Tamagotchis at the same time. I think it would have been more manageable had I only been running him at the time.
Despite how hard it was, it remains one of the most fun vintage Tamagotchis I've played with. It has really cute characters that move around the screen more uniquely than other ones, floating up and down or even swimming off-screen, and I love the little bubble effects added to help make it seem more watery.
Ultimately, it's one I'd love to run some more and experiment with, but not until I have the free time to do so.
Umi no Tamagotchi Overview
Umi no Tamagotchi Growth Chart