Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
seaside [ˈsiːsaɪd] n взмо́рье, примо́рье
to go to the seaside е́здить*е́хать*(пое́хать*perf) на взмо́рье
at the seaside на взмо́рье
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.