Yes, it can get unbearably hot as others have said - when we arrived in DXB at 3.00am last year it was 43 degrees C outside the terminal! I would not even consider using trains etc to get around - taxis are cheap and most places are strung out along either side of the multi-laned freeway so access is usually relatively direct. We even took a cab from DXB to Abu Dhabi which cost us 250 AED each way. At peak times like 8-10am travel times can blow out and it can be very difficult to get a cab and you'll likely be ignored if you try to hail one from the road - much better to wait outside your hotel and get the concierge to source one for you.
Regarding hotels, we have stayed at the Ocean View in the Marina Mall area - large, modern hotel with extensive food options available for breakfast. Had no complaints about the hotel itself although the pool was only average, but as a general comment I think we would stay closer to to the airport next time since most places you're likely to want to see will be up that end of the city. FWIW don't worry about hotels on the water - it will be so hot that you won't want to go anywhere near the beach..........but if you're after the views...............
Have also stayed at the Grand Hyatt and could definitely recommend it - great pools. There are so many very good hotel options that you will be spoiled for choice.
Afternoon Teas seem to be on everyone's "must do" lists in DXB and are probably something you will want to try but we wouldn't bother again. Traiteur Restaurant at the Park Hyatt is popular for Friday brunch - it was about $250 pp with champers for a buffet including lobster etc, but it's easy to over-eat. Probably would just go a la carte in future. To get in to the the Burj al Arab you need to be a guest or have a booking for afternoon tea etc. at the Skydeck Bar etc just to get in the front door. Didn't like the interior architecture of the hotel itself but a'noon tea was very nice - 7 courses and nice views, but our favourite, the Louis Roederer, was $AUD40 per glass! All the Malls are essentially the same so maybe just do the biggest, the Dubai Mall, and forget the others.