Good idea, but I would get professional advice with a pond that big. You should be able to fill with dirt and have a fantastic garden.

PS a bog does NOT smell unless you have lots of standing water.

This thread posted by TPN60 needs much more of a answer than can be written here, it is a complex question that needs detailed answers and I agree you need to get help in your area.
I am very informed on this subject, and you can certainly covert your pond to a bog. It will be expensive and the first thing you must realize is that drainage holes must be provided in the bottom or lower sides. You cannot have standing water at the bottom of the peat you will use. If you do you will have problems with many fungal disease and plant death as the roots drown. The stagnat water in the bottom not being able to drain will sour and even the best water will push minerals up towards the top slowly posioning the bog peat and plants. Drainage is a requirement.
Growing plants will use water, the roots of true bog plants will suck large amounts of water in the high heat of summer or in the spring when in active new growth. The sun and humidity will play a role in evaopration on the top and first couple of inches. Without drainage your bog may have a very wet bottom thats while the top peat is dried.

For a true bog you will need a large amount of Canadian sphagnum peat moss. The type sold at retailers in the square bales. This will cost a bunch unless you can get it wholesale. If you want a wetland garden it will still take a lot of humus mixed with acidic Canadian sphagnum peat moss and some blasting sand, not play sand.

Bogs are very acidic and a wide variety of plants can be grown in them but while the plants like wet roots they don't want to stand in water for long periods of time. Drainage is a must thus while reduction of watering maintenance is required care must be given not to let the bog dry out during drought or other dry periods of time.

Tap water is questionable but thats a lot of typing for now I will leave the topic alone.

You have a nice size area for a great bog garden. You will have at least the same maintenace in the summer as you have with the pond. Never forget that weeds love wet sunny areas and will quickly find a great home in a bog or wetland garden. Birds bring or drop seeds as well. Squirrels love to dig up plants and disprute bog gardens. The maintenance will be weed pulling and other typical garden procedures on a regular schedule or shortly you will a garden with a 5' high bunch of weeds, grasses, trees, and other non-desireable plants.

The deep end nearing 3' is going to be costly to fill. Typically a depth of 18 - 24" is a good depth for a bog garden.

No smell and I don't understand why your pond smells. This doesn't sound normal.

Plenty more, but this enough for now,

Take Care,

Mike
St. Petersburg FL