unlikely to be warning flags and lifeguards. The Mediterranean doesn't have a tide so less of an issue.
common sense sea safety - learn to swim, tell someone on shore where you are going, tell them when you get back, tell them what to do if you don't. If you hire kayaks from someone, they will obviously want the kayaks back so will have some interest in your return. Listen to their instructions about where you can and can't go and paddle INTO the wind - that way when you get tired, you get blown back to shore. Be aware that you will be given buoyancy aids, these are not the same as lifejackets. If it is too windy or bumpy, don't go.
sea urchins quite common, buy some water shoes for everyone.
4 x 4 not needed. Airport to Skala seems to be 35km so struggle to see it taking 2 hours, although if this is the holiday company bus there will be a certain amount of faffing.
for the plane - look up the rules for liquids, gels and pastes in hand luggage, they are clearly explained. For hold luggage, you can put in as much liquid etc as you like but wrap it well. Ditto aerosols. Nothing valuable or important in hold luggage. Top tip for planes: take an empty water bottle through security for each of you. Fill it from the tap (UK end) after security. except sodding Luton where you can't do this so they make you buy water