I am not sure why your greenhouse is "leaking", however you can raise electrical cords, especially at a junction, which can be sealed with some electrical tape to further keep water out. I have used common electrical wire (with insulation still on) bent into small hooks which act as hangers every fre feet, to raise cords off the floor. (Be sure to keep junctions higher, so any water that collects on them runs away from the junction & not to it.)
As for water/thermal mass, you can use smaller buckets, like that from cat litter. They are square in shape, which makes butting them together a nice space saving situation. (If you like, paint the side exposed to the sun black, to absorb more heat.) I use a ton of them to hold rainwater (for plant watering) over the winter, along with a couple rubbermaid troughs:
[
http://www.amazon.com/s/ref=nb_sb_n...aid+trough&rh=i:aps,k:rubbermaid+trough&ajr=0 ]
which come in a variety of sizes. In them I raise waterlilies and some other water plants. (I already have many Sarrs in other containers throughout the greenhouse.) If you have any shady areas (like along the bottom of the north wall of the greenhouse) you can place some insulated panels (painted pink foam or foil backed) along that wall to insulate dead areas where no useful light or heat will be entering anyway. (See below*.)
There are two factors that are most obviously involved in heating a greenhouse... they are
1. Getting the heat.... and
2. Keeping the heat where you want it!
So indeed, having the Greenhouse in the sun is great. But having it heat up thermal mass helps too! Making the most of the light & heat by putting a reflective surface on the walls where no sun enters will reflect it back to the plants or elsewhere inside the greenhouse. A layer of additional insulation there will also add to the "Keeping it where you want it".
If one side of the greenhouse is hit by wind, a barrier will help also (like low evergreen bushes), so long as it doesn't block out any sun.
*And if you want to save money in the long-run, be sure to insulate
as much as you can afford to. Having a piece of good foam insulation that runs a foot or two up from the ground all around the walls, will save a lot of heat and won't be blocking much sun at all. You can always keep it lower along the sun-facing wall, and higher on the North/shady side.
Depending on where you live, a small heater can help a lot. It can mean the difference between a nice collection & a lost one. Even then, it can be a risk, so having a back-up plan may be essential.
Just keeping the temps to a minimum amount may be enough, but look up all the plants you have to make sure the temps don't fall lower than all the plants individually need as their minimum.
By the way, I know it is kind of late now, but looking at the pic of your greenhouse floor, it seems that the flooring is already begining to be uneven. You may find it best to butt the brick close together, with slightly less than 1/16" of space between them. (You want just enough space to allow the thinnest layer of sand to set between the bricks. This will allow the brick to be more stable and have a good fit. Any excess sand (thickness) allows too much movement of the bricks, and not enough & the bricks won't "set". (You also want to use a very fine sand, which will fit into the tiniest of cracks.)
Any extra space (which I assume is why you chose to space the bricks out evenly as you did) should be filled in at the edges (or only along one wall/edge if not both) with a smaller brick run that fills up most of the remaining space. (It can be difficult to find the right width of brick, but they can be found if you are patient & look around.) This way the spacing between all the bricks can be the smallest possible. [An option is to pour limestone/paving screenings in the largest space & tamp it in well. This can tamp in almost as tight like concrete, which is what it is meant to do.
Anyway, this reduces any flooring movement, even when the flooring is repeatedly hosed down on hot summer days. Without it, you will eventually find the job likely needs to be redone in time anyway, so perhaps keep these tips in the back of your mind just in case.
I learned some of this by a little reading, watching & helping others, & a lot of experience myself. Seems I always had to do it first, before I learned a better way!
There is no "one/right" way to do things, so whatever you think is best to do, go for it!
We all have different conditions to deal with, and different things available to work with.
For me, it took a few years to get a handle on things, and to learn what the greenhouse does at different times of year.
Before I got a good heater, it could get below freezing inside. But then it could also get above 100 degrees too, when the sun was cooking in! Yes, in the middle of winter, with no other heat source! (In summer, it could get like an oven however!)
But now, with a variety of knowledge & a little equipment, I finally have somewhat of a handle on things.
I hope some of this may be of help to you. Good luck.
Paul