Well, firstly the Einsatzgruppen recorded detailed records of mass killings of jews on the battlefield in 1941-42, sending detailed records to various officials and SS officers, etc. A commander of one of these units testified that they killed 90,000 jews alone.
Rudolf Hoss, commander of Birkenau, wrote in his memoirs (written in Polish captivity after the war; he was also tortured, which is also written in his memoirs) that an order "all Jews without exception are to be destroyed." was received in 1941, and also mentions the gas chambers. Supporting eyewitness evidence also comes from SS guards and physicians acting at the camp.
Interestingly, Hoss mentions that Block 11 at Auschwitz - the preserved one - was abandoned early on as a gassing venue. The crematorium is mentioned in that article - what isn;t mentioned is that hydrocyanic acid traces were found in there by a study (
http://www.nizkor.org/hweb/orgs/polish/institute-for-forensic-research/). Said study compares the concentration of hydrocyanic acid in deloused blocs to the gas chambers.
With regards to the swimming pool at Auschwitz, AFAIK there isn't any evidence it was used by the inmates rather then the SS (the SS already played football on weekends in a ground behind the 2nd crematorium).
Certainly it had facilities for the guards to enjoy.
Of the aerial photos... a quick search revealed
this of a burning pit (for bodies)
With regards to use of the crematoria, there is a lot of support evidence for their gassing use; a written letter (from Jan 1943, from SS Captain Bischoff, head of the Auschwitz Central Construction Management to Berlin) makes specific use of the German for 'gas chamber' with regards to Crematoria 2. Orders also were made for 'gas tight' door and window seals (or record), and a work order specifically (again) refers to the chamber as a 'gas chamber'. The chambers were also unsuitable as air raid shelters - too small for the inmates, and over a kilometer from the guard huts. (there were also already 1/2 man air raid shelters dotted around the place, some of which still stand). Also, the glass in the window of the crematoria was protected against being broken
from the inside.
(obviously, there is testimony explicitly referring towards the use of Zyklon B from camp soldiers; Hans Stark, for example)
Furthermore, there are documents from civillian contractors referring to heating up the gas chambers - if this was a morgue as some claim, it would be a very bad thing to do. Good, however, for speeding up the vapourisation of Zyklon B (which was chucked in as solid state).
There are also documents which refer to Auschwitz' incineration rate being boosted to 120,000 bodies a month; if this was (for example) due to a typhus epidemic, it would mean 4/5ths of the camps projected population (150,000) would have to die and be replaced each month.
EDIT; took ages to find this
EDIT2; IIRC Auschwitz was originally built as a prisoner of war, then concentration/forced labour and finally execution camp; it wasn't originally designed for what it's famous for, hence why it's singled out as evidence 'against' because of incongrous design.
EDIT3; shifted to bottom; there's also other stuff about specific addition of gas proof seals on the crematoria and a paper which positively identified the gas holes there, but I couldn't be bothered typing the specifics.
EDIT4;
http://en.wikipedia.org/wiki/Witold_Pilecki Witold Pilecki, volunteered to go to Auschwitz to organise resistance, escaped to try and convice the Allies to send help in person. Claimed "During the first 3 years, at Auschwitz there perished 2 million people; in the next 2 years—3 million".
