Peel the shrimp, reserving the shells. (It is impractical to devein small shrimp, but if you are using the larger ones, I recommend that you do.) Split the shrimp lengthwise in half down the back, cover, and refrigerate until needed.
Remove the fronds (lacy top) from the fennel, coarsely chop, and reserve for garnish, covered and refrigerated. Trim off the tops and tough outer layers of the fennel and coarsely chop, reserve for the stock. Quarter the fennel bulb lengthwise, cut away the core, and cut very thinly across into 1/4- to 1/3-inch slices.
Add 2 tablespoons of the olive oil and the 2 crushed garlic cloves to a 10- to 12- inch high-sided skillet or sauté pan and heat over medium-high heat until the garlic begins to brown. Add the shrimp shells and sauté for 5 minutes, or until the shells turn red and are very aromatic. Add the fennel trimmings, the wine, and water, bring to a simmer, and simmer for 20 minutes. Season lightly with salt. Strain the stock; you should have 3 1/2 cups.
Heat a 4- to 6-quart heavy pot over low heat and add the bacon. Once it has rendered a few tablespoons of fat, increase the heat to medium and cook until the bacon is a crisp golden brown. Pour off all the fat except 1 tablespoon, leaving the bacon in the pot.
Add the remaining 1 tablespoon olive oil and chopped garlic and cook for 30 seconds. Add the onion, sliced fennel, fennel seeds, and optional crushed red pepper and sauté, stirring occasionally with a wooden spoon, for about 10 minutes, until the onion and fennel are softened but not browned.
Add the potatoes and the reserved shrimp stock. The stock should just barely cover the potatoes; if it doesn't, add enough water to cover. Turn up the heat and bring to a boil. Cover the pot and cook the potatoes vigorously for about 8 minutes, until they are soft on the outside but still firm in the center. If the broth hasn't thickened lightly, smash a few potatoes against the side of the pot and cook a minute or two longer to release their starch.
Reduce the heat to medium, add the tomatoes, and simmer for 5 minutes more. Stir in the shrimp and cook for 1 minute, then remove the pot from the heat and stir in the cream. Let sit for 10 minutes while the shrimp finish their slow cooking. Taste the chowder and season with black pepper and more salt if needed. If you are not serving the chowder within the hour, let it cool a bit, then refrigerate; cover the chowder after it has chilled completely. Otherwise, let it sit at room temperature for up to an hour, allowing the flavors to meld.
When ready to serve, reheat the chowder over low heat; don't let it boil. Ladle into cups or bowls, making sure that the shrimp, fennel, potatoes, and bacon are evenly divided. Sprinkle with the reserved chopped fennel fronds.