Did he set up the church himself, or is he just leading a church that is newly set up? If it's the former then that is unusual and possibly something to be wary of. But if it's the latter, then to be honest, what you've described sounds fairly mainstream.
Lots of churches (independents as well as mainstream denominations like baptist) do what is called a church plant. This is where about 30 or so people from a long-standing church leave together to set up a new church. This would typically be done either because the congregation is growing too big for the current venue or for example a big town centre church setting up a smaller church a local housing estate to provide for people there (e.g. toddler groups, elderly drop-ins). The new church would need to appoint a new leader/minister/pastor since the old one is still working in the original church.
Regarding baptising in swimming pools, this is also very common. Many independent churches (as well as some anglican and all baptist) practice adult baptism. Most small churches don't have their own pool, so would have the choice either to hire a large inflatable one or pop to the local swimming baths.
I can understand your concern. Most people's idea of church is the traditional c of e pews and bells etc. but a very significant number of churches are like the one your friend seems to have joined. To give you an idea, in the town I used to live (south east commuter belt) there were about 15-20 churches and nearly half were independent.
If you are concerned, there are a number of ways to tell if an independent church is "mainstream" or leaning towards bonkers cult. First of all, there are a few national umbrella organisations which most independent churches would sign up to (they don't want to be thought of as a cult either). The biggest is the Evangelical Alliance (don't be put off by the name, lots of c of e and other denominations are members of this too). On their website you can look up a town and see which churches are signed up to it. Also, many towns will have a "Churches Together" Organisation (typically called "Churches together in Bristol/Leeds/Maidstone/etc.) Is your friend's church a member of this? If so then that means that all the other member churches accept this church as legit.
In the town I used to live in, one of the churches went a bit wacky and cult like (named itself after its leader, discouraged contact with other churches, etc.) That church was removed from the local Churches Together group, who contacted the National Evangelical Alliance. They then visited the town and then struck it off their list, so was then obvious to all that this church was not the same as all the others.
I hope all this helps you (especially since it's taken me so long to type!)
I hope this helps