surf; waves breaking on the shore
Refers to waves crashing on a rocky shore or beach; a somewhat literary or poetic term.
磯波の音を聞きながら散歩した。
I took a walk while listening to the sound of the surf.
Compound of 磯 (iso, 'rocky shore') and 波 (nami, 'wave'). The reading いそなみ is a straightforward combination of the kun'yomi readings of the two kanji.