Does this happen if you use Walk in Shadow as well as Sneak?
More points in Sneak will reduce the sight cone of opponents, but if any can see you, it is normal that sneaking fails.
If you start a new game (explorer difficulty) with a default wizard and rogue character, and have the rogue cast Fast Track just before triggering the first combat encounter, they should have enough AP to get behind the opponents and sneak their first turn (have them in the lead, and keep to the left, to minimize the distance you would need to get around the summoned skeletons), and you can have the wizard cast Midnight Oil to slow the summons and maybe Boulder Bash to take out the 2 melee ones the next turn (after you test if the rogue unsneaks).
This should work without using Walk in Shadows; being invisible just guarantees opponents will not spot the character, even if they look in that direction.
Note that an autosave will be triggered when you move up the beach from the start, which could overwrite the oldest in your current playthrough. The number of quick and auto saves kept can be adjusted in the Game section of the options.
