Dreams involving gatherings with friends, particularly in familiar locations, can often symbolize your social connections and feelings of nostalgia. In your dream, the transformation of the location into a water park where you and your friends become mermaids may represent a desire for freedom, playfulness, or a return to a more carefree state of being. Mermaids often symbolize femininity, allure, and the exploration of emotions, suggesting that you might be embracing your own femininity or emotional depth in your current life.
The presence of your old high school friend might indicate a connection to your past, possibly reflecting unresolved feelings or memories from that time. Her joining the group could symbolize a blend of old and new relationships in your life, suggesting a merging of different social circles or aspects of your identity.
Overall, this dream may be encouraging you to explore your emotions and relationships more deeply, and to embrace both your past experiences and current friendships. It may also be a reminder to enjoy playful moments and reconnect with your inner child. Consider what aspects of your life feel like they could use a little more joy or freedom and how you can incorporate those feelings into your waking life.