We'll be on the coast early Oct. thru the 10th and back and forth on a ship part of the time. What would the dress be (shorts or long pants, tank tops or jackets)? I don't like being cold but also try to pack very light. Ideas?, Experiences?
Sometime between late September and the second week of October, the weather pattern is likely to shift between summer and fall. Go to wunderground.com and look for historical day-by-day weather data for your Croatian ports in 2016, 2015, etc. It got quite wet in Montenegro and Croatia by early October 2015, and it was beginning to get chilly in Zagreb. I would definitely want a warm layer (could be a lightweight fleece jacket or sweater), a waterproof jacket or poncho, and two pairs of slacks (unless I had one pair that would absolutely dry overnight). But I am cold-natured, and of course you might get lucky.
I was in Dubrovnik, Split, and Korcula in late September/early October of this past year and primarily wore short sleeves and light pants. In Korcula (from Sept 28-Oct 2) many people were still at the beach, and it was warm enough for this Midwesterner to swim. Like the previous post mentioned, it'd be a good idea to wear light layers, and perhaps bring a rain coat just in case. By the time we got to Zagreb (around October 4), it was very rainy and definitely cool enough for a sweaters and jackets.
I was on Hvar in 2015 and the weather broke 26 September, when it dropped from 25/27 degrees to 18/20 degrees and from sunshine to cloud and rain. Plan for a range of temperatures and some rain. There is a reason why many places there close at the end of September for the winter!
While the swimming beaches won't be very busy, and may no longer be renting beach chairs if there's been a cold spell, those that are shallow or in a protected bay should still be warm enough to swim. This has been my experience in many parts of the Mediterranean.
End of the season. If you will visit south locations Split Korcula and Dubrovnik than daily is still hot. You can count on whether change, so take one hot jumper. Croatians use jackets when they go out in publics. Light jacket will be fine for going out at evenings. In case you are visiting Plitvice Lakes than take 2 hot jumpers.
Have a good time!