You tried to ignore your throbbing erection, but soon your pants were uncomfortably tight. You couldn't take it any longer and stripped naked. The reason for your discomfort was obvious: your penis was two or three times its normal size. The shape was a little different and it had also turned gray. You touched it to make sure it wasn't turning necrotic. You could feel your touch alright and it was incredibly sensitive. You don't know marine biology very well, but you think it's like a dolphin. You had the biggest erection of your life and of course feel incredibly horny.
Two desires quickly dominate your mind: wanting to fuck and wanting to swim in the lake. Given the warning sign and the effects of drinking some of the lake water, you think the latter is a bad idea. As for the former: you wish your girlfriend would hurry back from gathering firewood. Then again, could your engorged cock even fit in her anymore?
You can't take it any longer. Finding a comfortable spot to sit with your back leaning against a large rock you begin to jerk off. Each stroke feels amazing! Finally a massive stream of cum erupts from you, splattering on the nearby rocks.
You lay there, breathing heavily. You feel good yet not completely fulfilled by your masturbation. And the desire to swim in the lake remains.
Just then our girlfriend returns with the firewood.