From your Barcelona apartment to Castelldefels

You can find a great Barcelona apartment if you like going to the beach as the city is located along the coast. However getting a Barcelona apartment near the beach in the city could be difficult due to it being peak season right now. Having said that, the beaches in the city can be overcrowded at Barceloneta and Vila Olimpica so it would be recommended to catch a train to somewhere like Castelledefels. It is a beautiful beach town located south of Barcelona about 30 minutes by train.
Instead of staying near the beach then maybe you could rent a Barcelona apartment near Sants or Passeig de Gracia train station.

Getting There
Depending on where you are staying, you can reach Sants Estacio by metro on the green line or blue line and Passeig de Gracia from the yellow, green or purple line.
Passeig de Gracia is a small station and is easy to find your direction however Sants is very big and a lot more complicated. From there, you head to platform (Via) 9 or 10 for all the trains heading south towards Tarragona.
You can either take the train heading for Vilanova, which stops at Castelldefels beach or take the St Vincent train which will only stop at Castelldefels town, where you will have to wait another 5/10 minutes for a train heading to the next stop which is the beach.
The price is 2.70 return and you can purchase a ticket from the many machines at the train station. Otherwise you can use your T10 ticket you would use on the metro in the city, as Castelldefels is within zone 1.
When you arrive at the stop, you are a mere 2 minutes walk from the beach, so instead of dirty, overcrowded beaches where you cannot relax, you can instead go to this beach where you have beautiful golden sand and clear blue sea. You should look for a Barcelona guide for more information.