Murano and burano how to

You can buy a day pass for the vaporetto at the ticket booth at St Marks right near the Doges Palace. Since individual tickets are pricey, and time limited you would have to purchase a new ticket for each leg of your journey so the day pass would be a good idea. The quickest way to get to Murano is to catch the vaporetto from the Fondementa Nove stop on the top of the island. It’s a short walk fro St. Marks, here is web site that shows it https://www.venicetraveltips.com/venice-to-murano/

If you want to see both islands I would recommend going to Burano first and then making your way back to Murano as there will be a lot of tourists making the same trip, and there will be a large crowd on Murano waiting for the vaporetto to go to Burano.

Anita's link identifies several vaporetto stops, including one in the San Marco area, where you can pick up a vaporetto to Murano/Burano. The boat trip will be longer from San Marco than from Fondamenta Nove, but Google Maps says the walk to Fondamenta Nove from the Doge's Palace will take 19 minutes (if you don't make navigational errors) and require you to cross up to 6 hump-backed bridges, so you might want to take the slower but slightly easier way. I don't know about the frequency of the vaporetti, though; it's possible you might run into a longer wait at San Marco than at Fondamenta Nove.

Murano is quite close to Venice proper; Burano is much farther away. I wouldn't normally recommend that island trip (which will take close to two hours for transportation alone) to someone with so little time in Venice. That said, Burano is very colorful. Murano is less physically attractive, but of course there's the glass.

Should I buy a pass for the water taxis?

Just to be clear on terminology, a water taxi is a private boat, like a land taxi to take you where ever you want. It costs about 100 EUR minimum to go anywhere. A vaporetto is a scheduled public boat, like a city bus. They cost 9.50 EUR per ride, unless you have purchased a pass.

Prices of vaporetto prices are as follows:

24 hours: 25 euros
48 hours: 35 euros
72 hours: 45 euros
168 hours (7 days): 65 euros

Note that it is not by the calendar day, but the number of hours for the passes

note that when using a vaporetto pass you still have to validate by tapping your pass before each boarding - very easy and necessary