how do I make my own Yu-Gi-Oh deck?

How many of each type of card heavily relies on what type of deck you want to play. The thing about Yugioh nowadays is that the decks mostly revolve around archetypes, which are basically related cards that form a group that mostly revolves around the same strategy. For example, a Masked HERO deck will only run about 10 monsters or so, about 20-25 spells, and about 10 or less traps, whereas a Lightsworn deck will run over 20-30 monsters and very few spells and traps. The Fire Fist deck that I play runs about 12-14 monsters, about 15 or so spells, and then about 12 traps. Card counts are not universal and depend on what archetype you decide to play.

Since you like water decks, I would recommend a Mermail deck. Mermails aren't the best water deck anymore, but they're not a bad deck from what I've heard. I'm not entirely familiar with how Mermail decks are built, unfortunately, but there are plenty of videos on youtube about Mermails that you can search up and learn about how the deck is built and played, since the deck used to be really popular. This deck could be a challenge, however, since it is a relatively competitive deck.