Getting to the beach from Havana is really easy and really cheap. I went to the beach twice while in Havana for 5cuc round trip which is about 5 dollars. You catch the above bus (the T3) in Parque Central.
The bus comes every 30 minutes and you pay in cash on the bus in cuc (you can only pay in cuc). The ride is about 30 minutes, the bus is air conditioned and the seats are comfortable. There is a guide that speaks in English and Spanish so you can ask her which stop to exit for the beach (I think there are 2 stops for you to choose from).
When you pay your 5 cuc you are given a receipt. Do not lose that receipt as that is your proof that you have already paid. If you lose it you will have to buy another ticket which is only good until the end of the day, NOT a full 24 hours from when you bought it.
The bus stop to go back to Havana is located in front of the hotel above. The bus comes every hour, please ask the guide what time the last bus is from the beach.
I wouldn’t recommend taking a taxi, it would cost you much more and there is no reason to when you can hop on the T3 bus.