Coastal marine heatwaves (MHWs) derived from satellite and blended sea surface temperature (SST) products often disagree, but the magnitude and drivers of this uncertainty remain poorly quantified. Using four daily SST products from 1982 to 2023, we quantify agreement at 9,946 global coastal points using two inter-product intersection-over-union metrics, a daily-scale co-identification rate (γ₁ ) and an event-scale rate (γ₂ ) based on overlapping event intervals. We find low global agreement, with daily- and event-scale rates averaging 12.34 ± 7.84% and 16.33 ± 8.35%, respectively, and interquartile ranges of 6.34%–17.19% and 10.17%–21.55%. Both γ₁  and γ₂  increase systematically with event duration and annual cumulative intensity. MHW trends are generally consistent across products, whereas trend attribution ratios show substantial inter-product divergence. These results indicate comparatively consistent estimates of long-term change but highlight the need to explicitly report multi-product uncertainty in coastal attribution.