How many points per heart

It is still possible to reach 10 hearts with all other Stardew Valley residents, which does not require giving them a bouquet. After marriage, your significant other will be able to have up to 13 of 12 hearts.

Oddly enough, you are still able to give bouquets to other marriageable candidates after marrying with no direct consequences that I've been able to observe.

It takes two days to upgrade the watering can. Since you can use the watering can both on the day you take it into the shop and the day you pick it up, there's normally only one day that you won't be able to water your plants. So you want to ensure you don't need to water your plants that day. There are a few easy ways to ensure this without having to resort installing a bunch of sprinklers.

• Upgrade the watering can on a day when the weather forecast for the next day is for rain
• Upgrade it on Spring 27, Fall 27 or Winter 27. You don't need to water your plants on the 28th day of these months because none of your plants will survive the change in seasons. Summer is an exception because corn will survive into Fall.
• Upgrade it on any day in the Winter (except Winter 28) if you're not growing one of the few crops, all Winter forage, that can grow then.

Finally you should also ensure that you'll be able to pick up the watering can on day the it will be available. Clint's store is closed or otherwise inaccessible on most festival days.

I should also I point out I discovered the last problem the hard way. As it turns it out not watering your crops one day isn't the end of the world. None of them died, it just delayed their growth by one day.