Because a degree in 50 years over the whole planet means something you're lipidinous blob of grey bias doesn't seem to fathom. And, of course, the rate of increase is accelerating.Sea surface temperatures can be measured from space ... but going down and measuring these parameters through the entire water column isn't all that common, and what data we do have is from the recent past ...
Today, that's different ... and every day we're dropping more equipment and instruments into the seas ... and we're gathering the data we need to evaluate this ...
The oceans can't warm "strongly" as you suggest unless the atmosphere is warming "strongly" ... we're at a degree every fifty years ... that's not noticeable ... who cares if today is 0.02ºC warmer than last year? ...