Public Transport Victoria publish the Victorian Fares and Ticketing Manual, which explains in depth how ticketing and fares work.
In particular, with respect to the myki Explorer card, this is simply a $6 myki Smartcard with $9 myki Money preloaded on it. (Locals pay $6 for a new card, too.) This is intended to be sufficient to cover the weekday fare cap for travel in Zones 1 and 2, which is what they consider Melbourne and its suburbs.
A myki Explorer is primarily designed to suit tourists and visitors to Melbourne and Victoria. The pack comprises a myki Smartcard, an amount of pre-loaded myki Money (providing one day’s travel in Zone 1 + 2), instructions on how to use and manage the myki Smartcard, an inner Melbourne map, a protective wallet, and discount vouchers for popular visitor attractions. It is available from selected hotels and visitor accommodation, and the PTV Hubs at Southern Cross Station and 750 Collins Street, Docklands.
The manual shows the daily fare cap for weekdays as $8.80 and weekends and public holidays as $6.30. If you use the card for more than two hours on a weekday you will end up with a balance of 20 cents on the card.
In other words, it is not a special type of one day pass, but simply enough myki Money to cover the maximum fare in Zone 1 and 2 for one day.
St. Kilda is in Zone 1, as is the airport. PTV consider anything in Melbourne and its suburbs in Zone 1 or 2 and you would be covered there. Anything outside these zones is considered "Regional" and is so indicated on the system maps.
With regard to SkyBus, the manual states that myki is not accepted.
V/Line tickets, myki, free travel passes and concessions are not accepted by SkyBus.
You can purchase SkyBus tickets online or at the ticket booth at the airport.
As far as I can tell the myki Explorer pack is the only card you can buy at the airport itself. Supposedly there were ticket machines there in the past, but they seem to have gone missing. If you have an Android phone with NFC and Google Pay, you can add Mobile myki to it and save the $6 cost of the physical card, then touch on and off with your phone. But you also won't get the tourist attraction vouchers this way. You can also buy one onboard the bus.