We honestly don't know if it all being in the tubby would be safer. Soap can only do so much as in kill what is on the skin surface. Of course being naked and covered in soap suds could be a hell of a lot of fun....he he he
So what do you do? Well if you don't know your partner - then it's an implied risk in general don't you think? You either accept it and do what you can to keep it minimal or you don't engage in it. Keeping your briefs on for it would lower your risk but obviously takes away from some of the pleasure that is part of outercourse in general. Wearing a condom even though there's no penetration would help with some std's but not all of them. You can talk to your partner prior about std's but if it's a random hook up, no guarantee's they'll be honest ( well that goes for any relationship but I hope you know what I mean ). hpv and herpes would be your highest risk from this but even at that the risk is pretty low in general. You aren't going to contract hiv without blood or body fluid exchange.
grace
i dont think using soap or any shampoo to prevent the virus from entering the body is a nice idea.. cause a virus would be not that simple to be killed by any soap or shampoo coz,, once the infected person had any contaxt such as having sex, it'll be a higher incidence of being infected also.
usually the transfer comes from having contact or exposure with infected person through there mucus membranes, genitals, blood.. and other body secretions but except the tears, sweat and saliva..
one thing that can lessen having STD is using condoms,
though it only lessen the possibility but they can acquire it also if they tried to have oral sex.
the only thing that can prevent having STD is having a monogamous relationship. not trying to have sex with other person you didn't know.