I worked there year year as divemaster for 6 months so if you want any details feel free to PM me.
St Nikolas beach only really do try dives. 40-50 a day sometimes more.
They do occasionally have shore dives but its the same walk off the beach, same bit of rock and back - not worth it in my opinion. Its a little spit of land near Vasilikos which is possibly the nicest area on the entire island but just a shallow shelving sandy bottom next to some rocks. A LONG way from the other dive sites.
Most of the islands 10 or so legal dive sites are in the Keri peninsula and as a result the dive operators tend to be in Laganas, Kalamaki, Keri areas for short boat journeys.
Diving you can expect good vis but little in the way of life - its all been eaten. Despite a marine park long lined trawling and even dynamite fishing is still practiced by the locals out of season along with spear fishing anything that moves.
You get generally walls, rocky reef and a few caves (which are the better dives).
Visibility usually 30m plus, the sea calm. Occasionally you get some currents (one site in particular is nasty for this).
Of the operators i can think of the Divers Paradise (in the louis hotel in Kalamaki, mainly caters to Germans. Has been renamed for this year), Zante Divers (Germans and mix of nationalities), Eurodivers (British and Scandinavian mainly) and Adventure Divers (Dutch, German and misc. nationalities) all on Laganas beach. Keri mainly caters towards Germans and there are turtle beach, nero and other centres there.
Some will offer pickups from other places on the island as the island itself is very small. One i know of has a sub shop in Tsilivi to arrange transport ('cos i worked there last year).
Basically it depends where you're staying on the island and what transport you have really.
Although Greek laws in theory have changed to legalise night diving, wreck diving and so on the reality on the ground is that the changes haven't filtered through so its still the same 10 sanitised sites there always have been and the coastguard actively pursue and fine anyone out of those areas (or take bribes...).
4 dives (2 days) of Zante should cover it. Just do the Keri Caves (the deep cave in particular), the butterfly cave, the arch and maybe Marathonisi.
Diving aside its a lovely island with great people and fantastic scenery so well worth a visit even without diving. Be sure to see Keri lighthouse at sunset, visit Limnionas and the taverna with great views, Kampi on the west coast for scenery and quiet bays, Agios Nikolaos in the NORTH (not to be confused with the one in the south that does try dives). Also you can drive to the mainland (or organised trip) via ferry and go visit Olympia (45 mins from ferry port) which is well worth it. Delfi is further but also worth it on the mainland.
If you want quiet then stay in Vasilikos and visit Gerakas, Dafni and other beaches. A bit more lively is Kalamaki then for wall to wall vomit its Laganas.
On the east coast nice bustling family resort of Tsilivi and a few further north.
If you're in Alykanas then your best bet would be to go to Tsilivi (10-15 mins in car) and use the little outpost shop there to get you to and from the main base in Laganas for diving - all the others would involve you driving to Laganas or Kalamaki yourself. St Nikolas beach is about 50 mins from there mainly due to the slow speed you need on the winding road.
Tsilivi to Lanagas my record was 12 minutes but better allowing 20 for a more sane speed.
There isnt anything in Alykanas or Alykes itself as there really is no diving that side of the island.
Im a bit reluctant to mention the name of the place in Tsilivi as i worked for them last year and it maybe against the board rules. Can supply info in a PM if required though.
Yes Ive been there. Very mellow and chilled out. We hired a car to the south of the Island. What a laugh. Anyway we ended up in Laganass. Dive centres all along the sea front. I dived with Euro Drivers. Padi most probably great fun. Great rock/wall dive not much fish but very clear water.. I think the locals have fished it out.. Great people friendly dive centre. They have turtles there if you time it right. Some of the beech is off limits so I was told. At laying and hatching times.
Being underwater and hearing dynamite fishing is quite frightening even when its a long way off.
As for turtles, June/July is the best time as they're coming in to lay their eggs. Usually large numbers in the bay of Laganas in the shallows only a few metres off the beach. A bit later in the year in main holiday season any turtle that dares show its head in the bay is usually instantly surrounded by 5 or 6 turtle spotting boats, random tour boats, snorkellers, pedalos and anything else that floats.
The eggs hatch really late August and September.
The beaches are mainly the bit between Kalamaki and Laganas (hence no building allowed behind it or lights). They dont close the beaches in daytime but do cover the more vulnerable nests with cages. Some are closed at night at certain times of year.
Further east, Sekania, Dafni and Gerakas are main turtle beaches (as is Marathonisi island).
Some areas of the bay all boat traffic is banned, others have various speed limits to avoid clouting the turtles. The coastguard ruthlessly enforce this (in the holiday season - its a free for all outside that).