Slice fillets into bite-size pieces; place in saucepan with water. Bring to a boil; reduce heat and simmer uncovered, for about 6 minutes or until fish flakes easily with a fork.
Remove fish with slotted spoon; set aside.
Add diced potato to water in the saucepan and cook, covered, for 5 to 6 minutes, or until just tender. You may need to add more water. Set aside, without draining.
In a large, heavy saucepan, melt butter, add celery, onion and seasonings. Fry until tender, for about 10 minutes; add flour, stirring over medium heat for 3 minutes, until mixture begins to bubble.
Meanwhile, in another saucepan, heat the cream and milk until hot (I put it in the microwave); do not allow to boil. Gradually add cream mixture to the flour mixture, stirring constantly until it begins to thicken, about 2 minutes.
Stir in potato and cooking water; add fish and lobster.
Return to the boiling point, and remove from heat.