Coastlines rhymes with:
dateline's, datelines, frontline's, hotlines, nightline's, nightlines, outlines, airline's, airlines, airlines', baselines, bloodlines, caroline's, dateline's, datelines, deadlines, enterline's, esterline's, felines, frontline's, guidelines, headlines, hemlines, hotlines, lifelines, moneyline's, nightline's, nightlines, online's, outlines, pipeline's, pipelines, pipelines', sidelines, skylines, smithkline's, storylines, strandlines, underlines, villines
Coastlines sounds like:
castellanos, cattlemen's, chitlins, coddling, cuddling