In case of some nuclear reactors, seawater is used as an emergency resource to remove the decay heat from the reactor core. This study aims to improve the understanding of boiling heat transfer with seawater coolants. Under the boiling conditions with seawater, the mass transfer of the dissolved impurities to the heated surface is expected to significantly impact the heat transfer characteristics. The focus of this experimental work is to measure the differences of the heat transfer performance between seawater and tapwater with electrically heated cylindrical section in a vertical annulus. High speed visualization is performed to quantify and characterize the bubble dynamics parameters. The experimental results indicate an enhanced heat transfer coefficient with seawater in the initial transient under saturated boiling followed by an asymptotic reduction to values similar to tapwater. Under subcooled boiling, a consistent reduced heat transfer coefficient was observed in seawater for a range of heat fluxes. The high speed visualization of subcooled boiling showed fewer and smaller bubbles nucleating off the heat transfer surface in seawater indicating a lower evaporative flux as compared to tapwater.