ΔE 30.6, loose: different enough that you should pick on purpose. Light Sea Blue is noticeably lighter, more muted, cooler than Summersea Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 31). Light Sea Blue is noticeably lighter, more muted, cooler than Summersea Blue.
The same difference applies the other way round: Summersea Blue is noticeably darker, more saturated, warmer.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Summersea Blue on the colour wheel See Light Sea Blue on the colour wheel
All matches: Summersea Blue equivalents · Light Sea Blue equivalents