Yes, there are public transports available to the beaches near Lisbon.
Are you aware of the Blue Flags certifications of beaches\marinas? Those are considered the best beaches\marinas in the country.
Even though there are no beaches in Lisbon, overall there are 15 beaches and one marina near Lisbon with blue flags certifications (Lisbon itself has one blue flagged marina, i often forget about it lol).
We can say that 14 of those beaches are closer to Lisbon, 10 in the northern margin (in Cascais), and 4 in the southern margin (in Almada), after the river. It's up to you to choose the beach, since some are for surfers, others are crowded, others are not, some are small and some are big, and so on.
To get to one of those 10 beaches in the northern margin, it's plain simple: you take the Cascais train line in Cais do Sodré station in Lisbon and buy a destination ticket anywhere between Carcavelos train station and Cascais train station. Those beaches are located in each single destination, though the main concentration is between S. João' s station and Cascais station (6 of them). The train line operates every day, all day long (Except from 1:31 am to 5:29 am), working days from monday to friday (except holidays, and weekends) and between morning and afternoon rush hours there are two services operating, one the full line, and the other the half line. The latter only goes until Oeiras so you take the full line that goes all the way until Cascais. Holidays and weekends there is only one service available that stops in every single station of the line.
The other 4 beaches are located in the southern margin, in Caparica, Almada, like i said, and it's a little more complicated to get there, though there are direct suburban buses from Lisbon (you need to go to Praça de Espanha to the terminal of TST buses, and take the bus nº153 - from July and all summer long there are one to three buses departing each hour all week long. Check local schedules, to choose a proper time). It takes about 40 minutes.
Besides this direct option you can also take the ferry and then take another bus in the southern margin, or the train and then another bus. If you prefer this one i'll give the options available in other post.
Can't help you on hotels, though.
