+1 on Little legends for long time wear, but to be honest all three of my kids passionately hated the big collar and restricted mobility at this age. Maybe your little one is more mellow
Also - they are tight for their stated weight - should not be a problem for your 7months old, but you'll graduate to next size earlier than you think - excellent snug fit though.
Also - it's not as comfy when it gets soaking wet, so I would not recommend swimming in it. If you want a swimming one, O'Neil or Stohlquist are better bets.
On a kayak you should not have an issue with a Stohlquist (or any neoprene PFD), but kids tend to get hotter in them than nylon shell ones. When we go sailing in hot weather I have my older kids sprinkle the youngest one with water
With all infant PFDs (ones with the collar) I recommend a dunk test in a pool to make sure that the fit is right and it will turn your kid face up.