QuestionIs it ok to skinny dip alone or with friends?
Technically you "skinny dip" each time you bathe. Being naked when you are by yourself is not an issue.
The general rule since the fall of man is that a person doesn't expose their genitals to others. We make efforts to keep our privates private. Of course there will always be times when it can't be avoided, yet we should make efforts to minimize it. Being seen naked in front of others ought to trigger a sense of shame. "Your nakedness shall be uncovered, yes, your shame will be seen" (Isaiah 47:3). One of the complaints God had against ancient Israel was the fact that they lost their sense of shame. ""Were they ashamed when they had committed abomination? No! They were not at all ashamed; nor did they know how to blush. Therefore they shall fall among those who fall; at the time I punish them, they shall be cast down," says the LORD" (Jeremiah 6:15; see also Jeremiah 8:12).
You also should be sensible about how your appearance affects other people as well. You might claim that seeing other people doesn't affect you. I would be inclined to argue that it does effect you since you ask this question. However, you cannot guarantee your friends won't be affected. "Therefore let us not judge one another anymore, but rather resolve this, not to put a stumbling block or a cause to fall in our brother's way" (Romans 14:13).