Watery sperm after aneros session?

Could be a partial ejaculation - seminal fluid mixed with 'precum', if it felt odd whilst you were ejaculating it probably wasn't happening quite as normal. Odd things happen I doubt it's anything to worry about.

Could have been a build up of semen mixed with prostate fluids. Have you cummed during this week, had sex with your partner? If you abstain from masturbation/sex for awhile, the same thing can happen.I get the same type of "feeling", particularly when I don't masturbate for a week and a half, or when i'm edging for nearly 2 hours straight, and then I release.
I would say that it is not something to really worry about, unless, of course, you get the same type of ejaculation and "feeling like peeing", everytime you ejaculate.

I go with the "excess" precum idea from @Smudgefish. Precum (from the Cowpers glands, downstream from the Prostate) is intended to ease penetration and 'grease the way" a shot of semen. If your urethera was well "greased" you might have diluted the semen a bit.

@Staley89_IT 's thoughts on excess Prostatic contribution may also have been a factor if you were using one of the longer Aneros toys like the PS-New. During a 'vigorous' session you may have milked the Vesicles down into the Prostate and that might have been a factor.

No reason for concern unless you do this just before you are trying to breed to foster a pregnancy. my2cworth. ... rook

I love that overflowing feeling you get when cumming from the prostate. Very different than a normal ejaculation.