In surveys Mason City residents rank water sports (swimming, boating, and fishing) among their favor

The proposed argument contains some stated and unstated assumption which are need to be checked for the validity of claim.
First "in survey mason city residents" here there is ambiguous reference to the survey. Not much information given about the survey like how many people included in the survey or was it a city wide survey or only restricted to the downtown area or suburb. It might be possible that the survey included very few people and so from that it is inferred that the Mason city residents like the water sports as recrearional activities.It may happen that if taken City wide survey the recreational acitvity might come diffrent,So wihout giving much information about the nature of the survey the claim made is unwarranted.

Also, their is no information for supporting the claim that Mason River is rarely used for the water recreational activities. It may be possible that in some areas river might little bit rough precluding any water sports related activity.But it might be possible that in some other parts of the ciy the river might be used for recreational sports or even for the transport.

There is no enought information provided about how much budget did the City park facility have for fiscal..It might be possible that the overall budget of the City park department might be low that's the reason why budget is allocated is less. It is not possible to determine whether the currently allocated budget is not sufficient for maintaining riverside recreational ativities,without the information about how the allocated budget is being used by the city Park department.

Argument also assumes that cleaning the river will increase the watersports level.Here, not enough information is provided to support the claim.It may be possible that even after the cleaning of the river the citizens may include only those groups included in the survey which will use the riverside recreational facilities.So, if that is the case then the investment made in for the river cleaning might not be recouped.

Another unstated assumption considered in the argument is that the river is potentially usable for the watersports.If the river flow is fast it won't be possible to conduct the watersports like boating fishing and swimming in the river.Also,river is not deep enough which will hobble the growth of these activities,So the unstated claim that river is usable for watersports needs to be supported with more information about the geographical structure of the part of river flowing through Mason City.

It is assumed that the city goverment already have additional funds available in order to increase the funding to city park department.It may be possible that even the state have announced the plans for cleaning the river city government but city government have already allocated funds to other pressing project and don't have enough funds available to increase the funding.Argument must support the claim by valid data about the remaining funds in the city givernement treasury or whether state governement haveprovided them with some additional funds.

Altough,Argument that state have anounced to clean the river is well supported with the information about the citizen's complaints about the smell and quality of water the other arguments made are not well supported with the information required to validate the argument.

Hello, I see that you need to check for 'number agreement'. That means you have to make sure to pluralize nouns that are supposed to be plural.

The proposed argument contains some stated and unstated assumption---- Make the word assumptions plural.

First, "in survey mason city residents"--- Is this the actual quotation from the source? It has a grammar error. It should say: In a survey of Mason city residents...

Another opportunity to improve your writing is here:
Also, their is no information... ---- This is an opportunity to practice the three kinds versions of this word: there, their, and they're. In this case, you need 'there' instead.

There is not enough information provided about how much the budget that the City park facility had ...

This is a very good sentence: It is not possible to determine whether the currently allocated...

When you write a sentence about a noun, you can begin the sentence with an 'article'. "The", "An, and "A"...

Argument also assumes that ... --- This sentence is wrong because it needs an article in front of the word 'Argument'.
So do this:
The argument also...

When you overcome a few small problems your English is going to be very impressive. You have strong command of advanced English grammar, but you made some mistakes that are easy to avoid. I think you got very good at some of the complex parts of English and you need to practice some basic sentence structures some more. Try writing short sentences sometimes. And also try mimicking the sentence structures you see in books and on websites; mimic the sentence structures you like.