Jackson Martínez opened the scoring with an overhead kick after 32 minutes and Varela put Porto firmly in control six minutes later.
James Rodríguez, who created the opening two goals, put the outcome beyond any doubt two minutes into the second half and Maicon added a fourth with 18 minutes remaining.
The win enabled Vítor Pereira's side to move three points clear of Benfica at the top of the table.
Benfica face Academica in Coimbra on Sunday.
Beira-Mar remain winless this season with a place in the relegation zone looming.