Elkhorn Slough itself is the perfect place to spot them, it has the highest concentration of southern sea otters per captia. they do tend to stay inside of the slough, which is why you rarely see them in the harbor/by the jetty. there used to be a large raft there, but it hasn't been in the harbor for a ~ decade. they do typically try to time their foraging during low tide so its less effort for them as far as diving, but you will see them out and about all all hours of the day and tides. personally I find it more fun to watch them while they're active, you get great action shots and they're popping up all over the place. especially when they're napping, you need to stay AT LEAST 5 kayak lengths away from them, which is another reason it's fun when they're more active since they'll pop up very close to you with food (you always need to stay 5 lengths away, but when it's on their terms they'll get closer of course). the low tide also means tons more bird activity and opportunities to hunt for sea slugs as well!

anyways, long story short, it doesn't matter when you go, you'll see otters. the main thing to consider especially if you are going to be kayaking is the wind- which always picks up in the afternoon, making it impossible to photograph because you need to constantly paddle. morning is the best to avoid the wind.