Congrats! I see we have something to look forward to. We are booked into Marina Santelena for 6 nights in early September. How strict are they about grey-water discharge in the "lagoon"? I have read that it is not permitted, which is not surprising...but then, this is Italy, where there are rules aplenty...and then reality ;-)
Officially you are not allowed to dump anything in the lagoon and I understand why, so we avoided that. Marina Santa Elena has showers, toilets etc, all in perfect working condition, hot water, cold water, so no need to use anything on the boat.
Inside the marina the water is calm, outside it is pretty rough due to all the boats racing around.
Best part is that, if you have a dinghy with an engine of less than, I think, 5 Hp, you don't need a special license for the dinghy, you can just go in. However, Grand Canal is always off limits for your boat and dinghy. But going through the canals with your dinghy is something special. I can advise to read up on the rules in Venice, they are slightly different in the canals.
One thing you will notice is that the rules on the water don't apply, everybody goes everywhere. Port to port ? Forget it. Everyone will just do their own thing, but the nice part is.............it works. The first moment you are overwhelmed with boats overtaking you left and right, crossing your path in front, behind and apparently no respect for the existing rules. However, then you realize that everyone simply avoids everyone, so nothing happens, nobody shouts or screams, they will find their way through all the boats, slow down, speed up when necessary, it is a bit like Naples, but then on the water. Once you get the hang of it you will love it.
Keep in mind there is a strong current in the lagoon. When the tides change you can have 4 to 5 knots current and that is interesting.
Also, on the electronic charts (navionics etc) the depths are not really correct. According to navionics the depth near Burano should be about 5 meters, we had less than 1 meter under the keel in certain places and you just don't know where the water is. You are searching for depth under the keel when it is low tide, so be careful.
If you still have time, try to order a paper chart for Venice, hopefully they have better depth information.
From Santa Elena you can have a nice walk into Venice, but you can also order a taxi, but they are expensive.
Other islands to visit are of course Murano, but parking your boat anywhere will be difficult. Near Burano en Torcella there is a canal where the depths are OK, but you don't have a lot of room to swing when the winds change. With the tide going in and out plus a strong wind you can have a lot of force on that anchor, so make sure you are anchored well.
Negative part of anchoring near Burano is that there are millions of mosquitos, so when you open the boat in the morning, you will have a welcoming committee. We did not have mosquitos in the marina though.
Being in Venice with your own boat is an experience not a lot of people will have. To be out on the water and have ancient Venice in the background is very special (at least to us).
As a tip, go see the popular tourist sites, but also go into the non touristic sites. Sit down at a local bar with the locals and have an Aperol spritzer, a glass of prosecco or a glass of wine/beer and simply enjoy the tranquility of a different Venice.
If you go to Torcello and you go with your own boat, go there after the tourists have gone. You will have the island almost to yourself. The first bar has a large garden, they serve wine from the tap and after all the tourists are gone the few locals gather there. I can recommend going there for a drink, prices are friendly.