My first experience watching girls while masturbating was on a house boat trip at Lake Meade California about ten years ago. We had a big group of friends out on a house boat trip for a week in July. Let me tell you it was hotter than hell during the day and everyone wore as little clothing as possible. Two or three of the girls on the boat were total hotties and between the sun tan oil and skimpy bikinis, I had all I could do to keep the wood at bay (if you know what I mean). Anyway, I had been sitting on the beach with the girls and listened as they exchanged stories about past boy friends, embarrassing moments, etc. After three days of constant exposure to these sexy creatures I was about to lose my mind. I excused myself to rid myself of some of the beverages I had been drinking and headed for the bathroom on the houseboat. The 'men's' bath was already occupied so I opted for the 'girls' bath at the front of the boat. As I began to relieve myself, I noticed I had a great view of the girls and they were probably no more than fifteen feet away.
At first I didn't think too much of it and finished my intended task, just then, I noticed the bikini Rosie had been wearing earlier that day, hanging on the towel rack. I couldn't resist inspecting it more closely; it was made of a black lycra material with a white liner, it was cool and still slightly damp from a swim earlier in the day. I found myself wondering if it would have any lingering 'scent' so I held her thong to my face and inhaled. I found only a faint smell of sun tan lotion and the lake. I found the liner of the thong was very soft and wondered what it might feel like to touch myself with it, I placed the inside liner of the bikini thong on the underside of my quickly hardening dick and slid it back and forth a few times and thought about how close it had been to her pussy only minutes before, it felt good! So good in fact, that if I continued much longer Rosie was going to find her thong a whole lot stiffer than she left it. I had already managed to leave a drop or two of pre-cum in the liner that had oozed from my aching cock during my initial exploration. The problem was that I was in no condition to return to the beach as I had about seven inches of raging hard-on in my shorts.
It occurred to me that I had started something that I was going to have to finish. On the sink was a bottle of tanning oil that Rosie and the rest of the girls had been using all during the trip. I poured a few drops on my hand and the smell of it put me back on the beach with them, their bodies flashed in my mind. I turned to the window and looked at Rosie's perfect tear drop shaped breasts and I could no longer resist the temptation to empty my aching balls. I began stroking myself slowly and deliberately with my freshly oiled hand. The silky smoothness of the oil allowed my hand to glide along the length of my shaft and I began to imagine what Rosie's lips and mouth might feel like on my throbbing cock. Before long I began to feel the deep spasms that foretold of imminent and copious ejaculation.
Just at that moment, Rosie called to me and asked if I would make them some drinks, not sure what to say, I answered that I was in the bathroom but would be right out. Of course, no one could resist the usual masturbation comments like; 'Shake it more than three times, you're playing with it' and 'will you be long?'
I studied Rosies perfect tits and slowly stroked my rock hard cock. I had to concentrate to steady my voice to answer Rosie's question, 'Nope, I'm about finished here...', then my body convulsed, with a grunt and a low moan, my thick, hot cum spattered the wall in long ropey gushes and rolled to the floor in thick clumps, eventually forming milky pools on the floor.
As I slowly regained my composure and my erection abated, it occurred to me that I had a hell of a mess to clean-up! I frantically wiped the wall and the floor with some toilet tissue, flushed it into the holding tank and returned to the beach. Later that afternoon as I passed the scene of the crime, I was shocked to see a fairly clear pattern, now apparently a stain on the bathroom wall.
A few days later, the trip was coming to an end, it was time to clean the boat before we turned it in. We all 'turned-to' in various areas of the boat and as expected, the girls took the bathrooms. I was working in the kitchen at the time and observed Rosie and Kim washing the wall and pondering what that stuff on the wall might be. I couldn't resist butting in to say 'It was there when we got on the boat'. 'I noticed it when we ran the checklist at the marina'. 'The deckhand and told me 'it was probably old spooge or something and not to worry about it'. A loud chorus of 'Eweew!', 'Gross!' and 'That's disgusting!' immediately followed along with flying wet sponges. I'm fairly sure that stain is there to this day.....