A california roll like you describe shouldn't be that bad, especially if they are using real crab. Some sushi places use a crab salad kind of thing with a creamy sauce that can really boost the calorie count. I actually love plain old imitation crab and eat it a lot - I never found it to be too salty! I love it in a spicy tomato pasta sauce. Avocados are high in fat/calories but they are good for you! 1 california roll a week shouldn't derail your progress, figure the calories into your day and enjoy!
If there is a Whole Foods around you - they offer California Rolls made with brown rice, just to make it a tiny bit healthier
I would be wary of spider rolls (fried crab), scallop sushi (usually made with a creamy sauce), unagi/eel (made with a sweet sauce) since they tend to be more high calorie options.
Here's a little calorie breakdown I found online:
Avocado roll: 246 calories, 11g fat, 33g carbs
Spicy tuna roll: 290 calories, 11g fat, 26g carbs
Shrimp tempura roll: 544 calories, 13g fat, 75g carbs
Philadelphia roll (salmon, cream cheese, avocado): 319 calories, 5g fat, 30g carbs
Spider roll (fried soft-shell crab): 317 calories, 12g fat, 38g carbs
California roll: 266 calories, 8.5g fat, 36g carbs
Cucumber roll: 136 calories, 0g fat, 30g carbs
Eel and avocado roll: 372 calories, 17.5g fat, 31g carbs
Tuna nigiri (2 pieces over rice): 240 calories, 1g fat, 27g carbs
Salmon sashimi (2 pieces, no rice): 164 calories, 6g fat, 0g carbs