It took me a little over 2 years to develop Pokettohiro. If I could have dedicated all my time to the project, it would have taken much less, but I had to work on it in my free time during the evenings. Sometimes it's hard to stay motivated for so long, but in the end, the result is worth it.
I agree with you! Ultimately, the limitations are part of the charm of these machines. People do amazing things with extremely limited resources.
If you'd like, you can follow me on Instagram (@pokettohiro) and the old Twitter (@Pokettohito_GBC). I'm not very active on social media, but when I have something important to share, I post it there :)
SPOILER
To enter the submarine, you need to complete a sort of "favor chain." You must buy the watermelon found in the shop in the cave that leads to the forest, then give it to Akane's hungry grandfather in the village. In gratitude, he'll give you another item that the girl at the submarine needs. I'd like to create a complete guide for the game soon, but I haven't had the time yet =_=
If you need more help during the game, you can message me directly on Instagram :)