Can an admin account on Mac access user's data?
A user with administrator privileges can read all your unencrypted files. And you have to assume the access to your data can happen without you knowing it (an administrator can simply take backup of the system and access the backed up files offline leaving no trace on the system).

One exception is the keychain file (the one containing all your saved passwords). It is encrypted and, by default, protected with the same password as the one you use for login. An administrator cannot access the contents and cannot change the password to this file without knowing the current one. Thus the administrator cannot access your saved passwords, credentials, and other data which you store in the Keychain etc.

The above applies only to passive "attack". A rogue administrator can install a key logger and simply learn your password by  sniffing