Meet the Singing Sailor, a "subsurface defense system" developed by The Swedish Peace and Arbitration Society.
The blinking neon sign reads "Welcome to Sweden" in Russian. The country legalized homosexuality in 1944.
Sweden recently announced plans to beef up its Navy amidst concerns about Russian activity in the Baltic Sea, but the activists proposed a different approach.
"If military actions and weapons had functioned as conflict-resolution methods there would be peace in the world a long time ago," Anna Ek, president of The Swedish Peace and Arbitration Society, said in a press release.
They placed the device underwater outside Stockholm. "Interesting info for any submarines passing close by," the group's website notes.
The device emits a message to nearby vessels in Morse code.
"If there is a submarine down there beneath the Baltic waves and the crew should happen to see or hear the Singing Sailor they are welcome to join us in the Stockholm Pride Parade on the 1st of August."