For markets, you can't do better than the famous Rialto Market, right in San Polo. When we were in Venice for a month, we went there 3 or 4 mornings a week to buy fish, fruit, and vegetables to cook in our apartment. It is great to walk around and just marvel at the beautiful displays of shellfish and crustaceans, and sometimes a really ugly whole fish whose name I cannot recall.
https://anamericaninrome.com/wp/2019/05/rialto-market-venice/
Some of the larger fish like swordfish and tuna are offered in cut portions, so you can point at the one you want if you don't speak Italian. Fruit can be purchased by the piece or by weight. Don't touch---ask for what you want. Around the edges of the area you will find small shops selling meats and cheeses.
Places to eat---yes, restaurant food is expensive in Venice. I will recommend the one place we really liked for reasonable prices, friendly service, and nice atmosphere, as well as good food: Rosa Rossa, on Calle de la Mandola in San Marco. It is described as a pizza restaurant, but they have other dishes as well (primi and secondi, as well as antipasti and desserts). It is somewhat famous as Johnny Depp's favorite place to frequent when he was in Venice in 2010, filming The Tourist.
https://restaurantguru.com/Rosa-Rossa-Venice
When you mentioned "water taxi" I think you mean the vaporetto, or "water bus" which is public transport around Venice and the other islands. Actual water taxis are private (they would take just the 4 of you wherever you want) and very expensive. The vaporetti are on regular schedules and call at various docks around Venice and the islands of Giudecca, Murano, Burano, and others. You MUST have a ticket or pass to board and in some cases to even be on the boarding dock. There are various types of tickets and passes and it does get complicated. Hopefully someone will have time to explain the system to you as I have to sign off for now . . .