There's not much of a visibility issue for kayaks here that I know of. Sounds like some safer routes should be marked out for them out of the traffic channels in congested areas.

I suppose we'll be seeing more paddleboarding off sailboats here in Maine as the number of those is going up quickly. But they're easy to see.

I find myself blocking traffic occasionally in my harbor rowing. You need the width for the oars so I go down the side of the channel. Facing the wrong way(rowing), I've turned around to see a big boat waiting to see what the hell I'm doing. No loud hailer yet though.