I see what you're saying. You're reading it as "every day is hotter than the previous day". The way I said it is open to misinterpretation but the article is better. It says "record-breaking year of heat" and "broken temperature records every single day over the past year," the record being the highest sea temperature for that day. Every day is an all time high for that day. The oceans are seasonally affected, so not constant temperature even averaged across the globe.

This is ignoring seasonality, of course Winter temperature can not be as high as summer temperatures but still break historic records (just as an example)

This is somehow a given when talking about seasonal data.