Sadly, without knowing the situation like this, Marisa's action could be misunderstood. Mister could've thought Marisa run away from responsibility after making a mistake and feared punishment.
Marisa is going out of the garden to look for a new home. Unless that sprinkler would cover the neighbourhood, she would be in safer condition compared to Reimu.
PS: since all the background is white without any sign, I'm surprised to see know that the sprinkler is set for 21:45 Is it common to water the plants at night ? I thought it's recommended to water them in the morning or late afternoon ^^;
If Reimu dies under the sprinkler before mister comes back, Marisa could always return to the shed and act like nothing happened, Mister probably thinks that either reimu learns her lesson and lives or dies outside being an idiot, so he may not pry further about the muddle of paste and dough that was reimu in his garden.