Hi - we go to Italy a lot so will try to help, the below is just my personal opinion and obviously others will have different views!
I assume you’re looking for somewhere on the coast if these 2 are coming up? I’ve not been to Viareggio but I have been to Lido de Jesolo. Bar Milan, it’s probably the place I’ve liked least in Italy. It’s basically a sand spit constructed by Mussolini as part of a ‘get back to work scheme’. There is a very long stretch of beach backed by a road then hotel after hotel. Quite a lot of these hotels were built in the 50’s/60’s & are well past their best (certainly the hell hole I stayed in was!). There are restaurants and a small park but compared to a lot of Italy it’s not especially attractive. Getting into Venice involves a bus journey to the ferry port then a ferry over. It’s a bit of a faff. If you are just looking to sit on a beach then it would be fine but there honestly are much much nicer places in Italy (IMO). It often comes up because it’s comparatively cheap compared to the Amalfi coast but it’s cheap for a reason.
If you are set on going to Venice there is an area called Lido which while I’ve not stayed there is supposed to be nice and getting into Venice itself is a much quicker journey. Various airlines do cheap flights into Venice and you could find a hotel (I’d go for an easy to cancel option!) in Lido on Booking.com
In terms of seaside alternatives, the Amalfi coast is gorgeous. It is more expensive than Lido Di Jesolo but if you DIY it (flights into Naples, Air BnB or Booking.com accommodation) you’ll save a couple of hundred. I personally love Amalfi. You can combine the seaside with a trip to Pompei.
Sanremo up in the North will cost you less - it’s on the Italian Riviera and used to be where the jet set went, it’s glamour has now faded but it’s got a lovely palm tree fringed promenade and an attractive Old Town. Flights into Nice and then either private transport or train it there (it’s about 1hr30).
Sicily is another nice option. Taormina (google it) is absolutely stunning with an old Greek amphitheatre and picturesque old town. Just off the coast is a little island (Isola Bella) you can snorkel round and walk too. You can also do a trip to the smoking volcano Etna from here. Flights into Catania (which is worth staying a night) and either a half hour train ride to Taormina or private transport.
It’s not the seaside but If you are happy to swap sea for Lakes, Lake Garda has some very pretty towns. They are easy to get too from Verona (which is gorgeous) either by bus or private transport.